Re: [PATCH] amdgcn: Add support for additional natively supported floating-point operations

[Tobias Burnus <tobias@codesourcery.com>]

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On 09.09.22 10:10, Andrew Stubbs wrote:
On 08.09.22 22:38, Kwok Cheung Yeung wrote:
The instructions for the transcendental functions are documented to have limited numerical precision, so they are only used if unsafe_math_optimizations are enabled for now.

-funsafe-math-optimizations implies -fno-signed-zeros, -fno-trapping-math, -fassociative-math,
and -freciprocal-math. All of them reduce precision and my violate IEEE or ISO/language standards.

However, I think it is rather surprising to have all of the sudden only a precision of the
order of 100,000,000 ULP instead of ~4 ULP as to be expected. That's a precision loss of the
order of 10^8 or 2^29 which is huge!

For program deliberately using double precision, it can be too much – even if they do not need
double precision in reality. (Weather forecast system recently moved to single precision as the
quality is similar and benefits of faster results/finer grids or longer forecast times prevail.)

As this behavior is highly surprising, I think it should be at least documented.

In https://gcc.gnu.org/PR105246 , I suggested a new flag (such as -mpermit-reduced-precision) to
make it possible turn it on/off explicitly (might be still enabled by -funsafe-math-optimizations);
alternatively, it could also be handled as initial guess for the result which is then refined
in some iteration steps. (It could also be combined to give the user the choice.)

While still being convinced that a flag makes more sense than just documenting it,
I have nonetheless attached a documentation attempt.

Thoughts?

Tobias


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GCN: Document in invoke.texi reduced precs with -funsafe-math-opt

gcc/ChangeLog:

	* doc/invoke.texi (AMD GCN Options): Document that
	-funsafe-math-optimizations implies single-precision
	results for some math intrinsics.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 5c066219a7d..01011bd9f9b 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -20139,8 +20139,14 @@ purpose.  The default is @option{-m1reg-none}.
 @cindex AMD GCN Options
 
 These options are defined specifically for the AMD GCN port.
 
+Note that the @option{-funsafe-math-optimizations} option implies that
+for 64bit floating-pointer numbers, the following operations yield results
+with only 23 bits instead of 52 bits for the fractional part of the
+floating-point number: @code{sqrt}, @code{exp2}, @code{log2}, @code{sin}
+and @code{cos}.
+
 @table @gcctabopt
 
 @item -march=@var{gpu}
 @opindex march