[FFmpeg-devel] [PATCH 2/2] Use gcc/clang builtins for av_sat_(add|sub)_64_c if available.

[Michael Niedermayer]

On Tue, May 26, 2020 at 12:23:41PM -0700, Dale Curtis wrote:
> On Fri, May 22, 2020 at 1:34 PM Michael Niedermayer <michael at niedermayer.cc>
> wrote:
> 
> >
> > does this produce faster / better code ?
> > that is do compilers actually fail to optimize this from plain
> > clean C code ?
> >
> 
> Here's the difference:
> clang trunk: https://godbolt.org/z/6SHmEo

interresting tool

but I tried it locally (plain C version with -O9)

av_sat_add64_c:                         # @av_sat_add64_c
	.cfi_startproc
	movabsq	$9223372036854775807, %rax # imm = 0x7FFFFFFFFFFFFFFF
	testq	%rsi, %rsi
	js	.LBB0_2
	movq	%rax, %rcx
	subq	%rsi, %rcx
	cmpq	%rdi, %rcx
	jg	.LBB0_2
	retq
.LBB0_2:
	addq	$1, %rax
	movq	%rax, %rcx
	subq	%rsi, %rcx
	leaq	(%rsi,%rdi), %rdx
	cmpq	%rdi, %rcx
	cmovlq	%rdx, %rax
	testq	%rsi, %rsi
	cmovgq	%rdx, %rax
	retq
.Lfunc_end0:
	.size	av_sat_add64_c, .Lfunc_end0-av_sat_add64_c
	.cfi_endproc
                                        # -- End function

	.ident	"clang version 9.0.0 (https://github.com/llvm/llvm-project/ 49b965079b18f8aa485dd1156dd088d40b7ee465)"

and the builtin one:
av_sat_add64_c:                         # @av_sat_add64_c
	.cfi_startproc
	leaq	(%rdi,%rsi), %rcx
	movabsq	$9223372036854775807, %rax # imm = 0x7FFFFFFFFFFFFFFF
	leaq	1(%rax), %rdx
	testq	%rcx, %rcx
	cmovnsq	%rdx, %rax
	addq	%rsi, %rdi
	cmovnoq	%rdi, %rax
	retq
	
so the builtin does better and both locally do better than clang on the web
tool



[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
they as if into a greater darkness enter who devote themselves
to the Knowledge alone. -- Isha Upanishad
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