On Tue, May 26, 2020 at 12:23:41PM -0700, Dale Curtis wrote: > On Fri, May 22, 2020 at 1:34 PM Michael Niedermayer <mich...@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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