From Newsgroup: comp.lang.c

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the code. What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.



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From Newsgroup: comp.lang.c

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the
code.
What happens in this case?

The solution I can think of is emulation when evaluating constant expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise. (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system. Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */
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From Newsgroup: comp.lang.c

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the
code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise. (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system. Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.


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From Newsgroup: comp.lang.c

On 2025-08-29, Thiago Adams <thiago.adams@gmail.com> wrote:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the code. What happens in this case?

A constant expression must be evaluated in the way that would happen
if it were translated to code on the target machine.

Thus, if necessary, the features of the target machine's arithmetic must
be simulated on the build machine.

(Modulo issues not relevant to the debate, like if the expression
has ambiguous evaluation orders that affect the result, or undefined
behaviors, they don't have to play out the same way under different
modes of processing in the same implementation.)

The solution I can think of is emulation when evaluating constant expressions. But I don't if any compiler is doing this way.

They have to; if a constant-folding optimization produces a different
result (in an expression which has no issue) that is then an incorrect optimization.

GCC uses arbitrary-precision libraries (GNU GMP for integer, and GNU
MPFR for floating-point), which are in part for this issue, I think.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca
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From Newsgroup: comp.lang.c

In article <20250829131023.130@kylheku.com>,
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-08-29, Thiago Adams <thiago.adams@gmail.com> wrote:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the code. >> What happens in this case?

A constant expression must be evaluated in the way that would happen
if it were translated to code on the target machine.

Thus, if necessary, the features of the target machine's arithmetic must
be simulated on the build machine.

(Modulo issues not relevant to the debate, like if the expression
has ambiguous evaluation orders that affect the result, or undefined >behaviors, they don't have to play out the same way under different
modes of processing in the same implementation.)

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

They have to; if a constant-folding optimization produces a different
result (in an expression which has no issue) that is then an incorrect >optimization.

GCC uses arbitrary-precision libraries (GNU GMP for integer, and GNU
MPFR for floating-point), which are in part for this issue, I think.

Dealing with integer arithmetic, boolean expressions, character
manipulation, and so on is often pretty straight-forward at to
handle for a given target system at compile time. The thing,
that throws a lot of systems off, is floating point: there exist
FPUs with different hardware characteristics, even within a
single architectural family, that can yield different results in
a way that is simply unknowable until runtime. A classic
example is hardware that uses 80-bit internal representations
for double-precision FP arithmetic, versus a 64-bit
representation. In that world, unless you know precisely what microarchitecture the program is going to run on, you just can't
make a "correct" decision at compile time at all in the general
case.

- Dan C.

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From Newsgroup: comp.lang.c

On 2025-08-29 16:19, Kaz Kylheku wrote:

On 2025-08-29, Thiago Adams <thiago.adams@gmail.com> wrote:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the code. >> What happens in this case?

A constant expression must be evaluated in the way that would happen
if it were translated to code on the target machine.

Thus, if necessary, the features of the target machine's arithmetic must
be simulated on the build machine.

The solution I can think of is emulation when evaluating constant

expressions. But I don't if any compiler is doing this way.

They have to; if a constant-folding optimization produces a different
result (in an expression which has no issue) that is then an incorrect optimization.

Emulation is necessary only if the value of the constant expression
changes which code is generated. If the value is simply used by the calculations, then the value can be calculated at run time on the target machine, as if done before the start of main().
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From Newsgroup: comp.lang.c

On 2025-08-30, James Kuyper <jameskuyper@alumni.caltech.edu> wrote:

On 2025-08-29 16:19, Kaz Kylheku wrote:

On 2025-08-29, Thiago Adams <thiago.adams@gmail.com> wrote:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the code.
What happens in this case?

A constant expression must be evaluated in the way that would happen
if it were translated to code on the target machine.

Thus, if necessary, the features of the target machine's arithmetic must
be simulated on the build machine.

The solution I can think of is emulation when evaluating constant

expressions. But I don't if any compiler is doing this way.

They have to; if a constant-folding optimization produces a different
result (in an expression which has no issue) that is then an incorrect
optimization.

Emulation is necessary only if the value of the constant expression
changes which code is generated. If the value is simply used by the calculations, then the value can be calculated at run time on the target machine, as if done before the start of main().

But since the former situations occur regularly (e.g. dead code elimination based on conditionals with constant test expressions) you will need to implement that target evaluation strategy anyway. Then, if you have it, why wouldn't you just use it for all constant expressions, and not have to make arrangements for load-time initializations.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca
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From Newsgroup: comp.lang.c

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the
code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system.  Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.

Yes, it can be hard to achieve in some cases. For things like integer arithmetic, it's no serious challenge - floating point is the biggie for
the challenge of getting the details correct when the host and the
target are different. (And even if the compiler is native, different
floating point options can lead to significantly different results.)

Compilers have to make sure they can do compile-time evaluation that is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.



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From Newsgroup: comp.lang.c

On 9/1/2025 5:10 AM, David Brown wrote:

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the >>>> code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system.  Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.

Yes, it can be hard to achieve in some cases.  For things like integer arithmetic, it's no serious challenge - floating point is the biggie for
the challenge of getting the details correct when the host and the
target are different.  (And even if the compiler is native, different floating point options can lead to significantly different results.)

Compilers have to make sure they can do compile-time evaluation that is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler.  I don't
know about other compilers, but gcc has a /huge/ library that is used to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

Interesting.
Yes, I think for integers it is not so difficult.
if the compiler has the range int8_t ... int64_t then it is just a
matter of selection the of fixed size according with the
abstract type for that platform.

For floating points I think at least for "desktop" computers the result
may be the same.



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From Newsgroup: comp.lang.c

On 9/1/2025 4:14 AM, Thiago Adams wrote:

On 9/1/2025 5:10 AM, David Brown wrote:

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the >>>>> code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system.  Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.

Yes, it can be hard to achieve in some cases.  For things like integer
arithmetic, it's no serious challenge - floating point is the biggie
for the challenge of getting the details correct when the host and the
target are different.  (And even if the compiler is native, different
floating point options can lead to significantly different results.)

Compilers have to make sure they can do compile-time evaluation that
is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler.  I don't
know about other compilers, but gcc has a /huge/ library that is used
to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

Interesting.
Yes, I think for integers it is not so difficult.
if the compiler has the range int8_t  ...  int64_t  then it is just a matter of selection the of fixed size according with the
abstract type for that platform.

For floating points I think at least for "desktop" computers the result
may be the same.

Think of a program that is sensitive to floating point errors...
Something like this crazy shit:

https://groups.google.com/g/comp.lang.c++/c/bB1wA4wvoFc/m/GdzmMd41AQAJ



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From Newsgroup: comp.lang.c

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation that
is bit-perfect to run-time evaluation before they can use it as an optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used
to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point emulation.
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */
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From Newsgroup: comp.lang.c

On 2025-09-01, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation that
is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used
to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point emulation.

GCC also uses not only GNU GMP but also GNU MPFR.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca
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From Newsgroup: comp.lang.c

On 01/09/2025 23:11, Keith Thompson wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation that
is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used
to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point emulation.

I am afraid I don't know the details here, and to what extent it is
internal to the GCC project or external. I /think/, but I could easily
be wrong, that general libraries like GMP are used for the actual calculations, while there is GCC-specific stuff to make sure things
match up with the target details.

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From Newsgroup: comp.lang.c

On Tue, 2 Sep 2025 06:40:43 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-01, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation
that is bit-perfect to run-time evaluation before they can use it
as an optimisation - any risk of an error is a bug in the
compiler. I don't know about other compilers, but gcc has a
/huge/ library that is used to simulate floating point on a wide
range of targets and options, precisely so that it can get this
right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point
emulation.

GCC also uses not only GNU GMP but also GNU MPFR.

MPFR is of no help when you want to emulate an exact behavior of
particular hardware format. More so, it can not even emulate an exact
behavior of IEEE-754 binary formats because it uses much wider range
of exponent than any of them. Even IEEE binary256 has only 19
exponent bits. OTOH AFAIR the range of exponent in MPFR can not be
reduced below 32 bits.

The problem is not dissimilar to inability of x87 FPU to emulate an
exact behavior of IEEE binary32 and binary64, because x87 arithmetic
OPs always use wider (16b) exponent.


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From Newsgroup: comp.lang.c

On 2025-09-02, Michael S <already5chosen@yahoo.com> wrote:

On Tue, 2 Sep 2025 06:40:43 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-01, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation
that is bit-perfect to run-time evaluation before they can use it
as an optimisation - any risk of an error is a bug in the
compiler. I don't know about other compilers, but gcc has a
/huge/ library that is used to simulate floating point on a wide
range of targets and options, precisely so that it can get this
right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point
emulation.

GCC also uses not only GNU GMP but also GNU MPFR.

MPFR is of no help when you want to emulate an exact behavior of
particular hardware format.

Then why is it there? I can easily see how such a library can be
used as the underlying framework for that kind of computation.

It provides the substrate in which an exact answer can be calculated in
a platform-independent way, and could then be coerced into a
platform-specific result according to the abstract rule by which the
platform reduces the abstract result to the actual one.

My hard-earned intuition (lovely oxymoron, ha!) says that this would be
easier than ad-hoc methods not using a floating point calculation
library.

Disclaimer: the preceding remarks are just conjecture, not based on
examining how MPFR is used in the GNU Compiler Collection.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca
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From Newsgroup: comp.lang.c

David Brown <david.brown@hesbynett.no> wrote:

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that runs the >>>> code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system.  Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.

Yes, it can be hard to achieve in some cases. For things like integer arithmetic, it's no serious challenge - floating point is the biggie for
the challenge of getting the details correct when the host and the
target are different. (And even if the compiler is native, different floating point options can lead to significantly different results.)

Compilers have to make sure they can do compile-time evaluation that is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

AFAIK in normal mode gcc do not consider differences between compile-time
and run-time evaluation of floating point constants as a bug. And
the may differ, with compile-time evaluation usually giving more
accuracy. OTOH they care vary much that cross-compiler and native
compiler produce the same results. So they do not use native
floating point arithmetic to evaluate constants. Rather, both
native compiler and cross complier use the same partable library
(that is MPFR). One can probably request more strict mode.
If it is available, then I do not know how it is done. One
possible approach is to delay anything non-trivial to runtime.
Non-trivial for floating point is likely mean transcendental
functions: different libraries almost surely will produce different
results and for legal reasons alone compiler can not assume access
to target library.

Ordinary four arithmetic operations for IEEE are easy: rounding is
handled by MPFR and things like overflow, infinities, etc, are just
a bunch of tediouis special cases. But tanscendental functions
usually do not have well specified rounding behaviour, so exact
rounding in MPFR is of no help when trying to reproduce results
from runtime libraries.

Old (and possibly some new) embedded targets are in a sense more
"interesting", as they implemented basic operations in software,
frequently taking some shortcuts to gain speed.
--
Waldek Hebisch
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From Newsgroup: comp.lang.c

On Mon, 1 Sep 2025 10:10:17 +0200, David Brown wrote:

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams writes:

My curiosity is the following:

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

...

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work correctly.

so in
So in theory it has to be the same result. This may be hard do achieve.

Yes, it can be hard to achieve in some cases. For things like integer >arithmetic, it's no serious challenge - floating point is the biggie for
the challenge of getting the details correct when the host and the
target are different.

float point is not ieee stardadizated?

(And even if the compiler is native, different
floating point options can lead to significantly different results.)

Compilers have to make sure they can do compile-time evaluation that is >bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used to >simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

i think there are few problems when one use a type of fixed size as
u32 or unsigned int32_t

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From Newsgroup: comp.lang.c

On Tue, 2 Sep 2025 16:58:45 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-02, Michael S <already5chosen@yahoo.com> wrote:

On Tue, 2 Sep 2025 06:40:43 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-01, Keith Thompson <Keith.S.Thompson+u@gmail.com>
wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation
that is bit-perfect to run-time evaluation before they can use
it as an optimisation - any risk of an error is a bug in the
compiler. I don't know about other compilers, but gcc has a
/huge/ library that is used to simulate floating point on a wide
range of targets and options, precisely so that it can get this
right.

What library are you referring to? Is it something internal to
gcc? I know gcc uses GMP, but that's not really floating-point
emulation.

GCC also uses not only GNU GMP but also GNU MPFR.

MPFR is of no help when you want to emulate an exact behavior of
particular hardware format.

Then why is it there?

Most likely because people that think that compilers make extraordinary
effort in order to match FP results evaluated at compile time with
those evaluated at run time do not know what they are talking about.
As suggested above by Waldek Hebisch, compilers are quite happy to do compile-time evaluation at higher (preferably much higher) precision
than at run time.


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From Newsgroup: comp.lang.c

On Tue, 2 Sep 2025 17:32:56 -0000 (UTC)
antispam@fricas.org (Waldek Hebisch) wrote:

David Brown <david.brown@hesbynett.no> wrote:

On 29/08/2025 22:10, Thiago Adams wrote:

Em 29/08/2025 16:54, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

My curiosity is the following:

Given a constant expression, the result maybe be different in the
machine that compiles the code compared with the machine that
runs the code.
What happens in this case?

The solution I can think of is emulation when evaluating constant
expressions. But I don't if any compiler is doing this way.

For example, 65535u + 1u will evaluate to 0u if the target system
has 16-bit int, 65536u otherwise.  (I picked an example that
doesn't depend on UINT_MAX or any other macros defined in the
standard library.)

yes this is the kind of sample I had in mind.

Any compiler, cross- or not, is required to evaluate constant
expressions correctly for the target system.  Whether they do so
by some sort of emulation is an implementation detail.

Even a non-cross compiler might not be implemented in exactly
the same language and configuration as the code it's compiling,
so evaluating constant expressions locally might not work
correctly.

so in
So in theory it has to be the same result. This may be hard do
achieve.

Yes, it can be hard to achieve in some cases. For things like
integer arithmetic, it's no serious challenge - floating point is
the biggie for the challenge of getting the details correct when
the host and the target are different. (And even if the compiler
is native, different floating point options can lead to
significantly different results.)

Compilers have to make sure they can do compile-time evaluation
that is bit-perfect to run-time evaluation before they can use it
as an optimisation - any risk of an error is a bug in the compiler.
I don't know about other compilers, but gcc has a /huge/ library
that is used to simulate floating point on a wide range of targets
and options, precisely so that it can get this right.

AFAIK in normal mode gcc do not consider differences between
compile-time and run-time evaluation of floating point constants as a
bug. And the may differ, with compile-time evaluation usually giving
more accuracy. OTOH they care vary much that cross-compiler and
native compiler produce the same results.

For majority of "interesting" targets native compilers do not exist.

So they do not use native
floating point arithmetic to evaluate constants. Rather, both
native compiler and cross complier use the same partable library
(that is MPFR). One can probably request more strict mode.
If it is available, then I do not know how it is done. One
possible approach is to delay anything non-trivial to runtime.

I certainly would not be happy if compiler that I am using for embedded
targets that typically do not have hardware support for 'double' will
fail to evaluate DP constant expressions in compile time.
Luckily, it never happens.

Non-trivial for floating point is likely mean transcendental
functions: different libraries almost surely will produce different
results and for legal reasons alone compiler can not assume access
to target library.

Right now in C, including C23, transcendental functions can not be parts
of constant expression.

Ordinary four arithmetic operations for IEEE are easy: rounding is
handled by MPFR and things like overflow, infinities, etc, are just
a bunch of tediouis special cases. But tanscendental functions
usually do not have well specified rounding behaviour, so exact
rounding in MPFR is of no help when trying to reproduce results
from runtime libraries.

Old (and possibly some new) embedded targets are in a sense more "interesting", as they implemented basic operations in software,
frequently taking some shortcuts to gain speed.

Why "some new"? Ovewhelming majority of microcontrollers, both old and
new, do not implement double precision FP math in hardware.
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From Newsgroup: comp.lang.c

On 02/09/2025 21:40, Michael S wrote:

On Tue, 2 Sep 2025 16:58:45 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-02, Michael S <already5chosen@yahoo.com> wrote:

On Tue, 2 Sep 2025 06:40:43 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-09-01, Keith Thompson <Keith.S.Thompson+u@gmail.com>
wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation
that is bit-perfect to run-time evaluation before they can use
it as an optimisation - any risk of an error is a bug in the
compiler. I don't know about other compilers, but gcc has a
/huge/ library that is used to simulate floating point on a wide
range of targets and options, precisely so that it can get this
right.

What library are you referring to? Is it something internal to
gcc? I know gcc uses GMP, but that's not really floating-point
emulation.

GCC also uses not only GNU GMP but also GNU MPFR.

MPFR is of no help when you want to emulate an exact behavior of
particular hardware format.

Then why is it there?

Most likely because people that think that compilers make extraordinary effort in order to match FP results evaluated at compile time with
those evaluated at run time do not know what they are talking about.
As suggested above by Waldek Hebisch, compilers are quite happy to do compile-time evaluation at higher (preferably much higher) precision
than at run time.

Doing the calculations at higher precision does not necessarily help.

If a run-time calculation can give slightly inaccurate results because
the rounding errors happen to build up that way, a compile-time
calculation has to replicate that if it is a valid optimisation.

I don't know the details of how GCC achieves this, but I /do/ know that
a significant body of code is used to get this right - across widely
different targets, and supporting a range of different floating point
options. If the compiler can't see how to get it bit-perfect at compile
time, it has to do the calculation at run-time.


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From Newsgroup: comp.lang.c

David Brown <david.brown@hesbynett.no> wrote:

On 01/09/2025 23:11, Keith Thompson wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

Compilers have to make sure they can do compile-time evaluation that
is bit-perfect to run-time evaluation before they can use it as an
optimisation - any risk of an error is a bug in the compiler. I don't
know about other compilers, but gcc has a /huge/ library that is used
to simulate floating point on a wide range of targets and options,
precisely so that it can get this right.

What library are you referring to? Is it something internal to gcc?
I know gcc uses GMP, but that's not really floating-point emulation.

I am afraid I don't know the details here, and to what extent it is
internal to the GCC project or external. I /think/, but I could easily
be wrong, that general libraries like GMP are used for the actual calculations, while there is GCC-specific stuff to make sure things
match up with the target details.

Indeed. . There's emulation for everything, even decimal floating
point.

See https://github.com/gcc-mirror/gcc/blob/master/gcc/real.cc

Andrew.
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