[cfe-dev] "Optimized implementations"?
Craig Topper via cfe-dev
cfe-dev at lists.llvm.org
Wed Sep 9 09:25:31 PDT 2020
The code in the builtins library is written in C and can't use
__builtin_parity since it needs to compile with compilers that don't have
__builtin_parity like MSVC. So to get the optimized code, we would have to
have a version written in assembly for X86. Probably two assembly versions
since MASM and gcc use different assembly syntax. So we'd have 3 different
versions of parity for 3 different bit widths. Maybe some macros could
allow us to share some bit widths or something. But ultimately it was a
question of where to spend effort.
~Craig
On Wed, Sep 9, 2020 at 9:08 AM Stefan Kanthak <stefan.kanthak at nexgo.de>
wrote:
> "Craig Topper" <craig.topper at gmail.com> wrote:
>
> > Turn on optimizations.
>
> What's the (deeper) reason NOT always to generate the optimised code for
> __builtins_*?
> Alternative question: do you also ship a library with unoptimised code
> which
> is linked when the -O option is not given?
>
> Just wondering about surprising behaviour
> Stefan
>
> > On Wed, Sep 9, 2020 at 4:28 AM Stefan Kanthak <stefan.kanthak at nexgo.de>
> > wrote:
> >
> >> "Craig Topper" <craig.topper at gmail.com> wrote:
> >>
> >> > __builtin_parity uses setnp on older x86 and popcnt with sse4.2
> >>
> >> Reality check, PLEASE:
> >>
> >> --- bug.c ---
> >> int main(int argc, char *argv[]) {
> >> return __builtin_parity(argc);
> >> }
> >> --- EOF ---
> >>
> >> clang -o- -target i386-pc-linux -S bug.c
> >> clang version 10.0.0
> >> Target: i386-pc-linux
> >>
> >> pushl %ebp
> >> movl %esp, %ebp
> >> subl $8, %esp
> >> movl 8(%ebp), %eax
> >> movl $0, -4(%ebp)
> >> movl 8(%ebp), %ecx
> >> movl %ecx, %edx
> >> shrl %edx
> >> andl $1431655765, %edx # imm = 0x55555555
> >> subl %edx, %ecx
> >> movl %ecx, %edx
> >> andl $858993459, %edx # imm = 0x33333333
> >> shrl $2, %ecx
> >> andl $858993459, %ecx # imm = 0x33333333
> >> addl %ecx, %edx
> >> movl %edx, %ecx
> >> shrl $4, %ecx
> >> addl %ecx, %edx
> >> andl $252645135, %edx # imm = 0xF0F0F0F
> >> imull $16843009, %edx, %ecx # imm = 0x1010101
> >> shrl $24, %ecx
> >> andl $1, %ecx
> >> movl %eax, -8(%ebp) # 4-byte Spill
> >> movl %ecx, %eax
> >> addl $8, %esp
> >> popl %ebp
> >> retl
> >>
> >>
> >> clang -o- -target amd64-pc-linux -S bug.c
> >>
> >> pushq %rbp
> >> .cfi_def_cfa_offset 16
> >> .cfi_offset %rbp, -16
> >> movq %rsp, %rbp
> >> .cfi_def_cfa_register %rbp
> >> movl $0, -4(%rbp)
> >> movl %edi, -8(%rbp)
> >> movl -8(%rbp), %eax
> >> movl %eax, %ecx
> >> shrl %ecx
> >> andl $1431655765, %ecx # imm = 0x55555555
> >> subl %ecx, %eax
> >> movl %eax, %ecx
> >> andl $858993459, %ecx # imm = 0x33333333
> >> shrl $2, %eax
> >> andl $858993459, %eax # imm = 0x33333333
> >> addl %eax, %ecx
> >> movl %ecx, %eax
> >> shrl $4, %eax
> >> addl %eax, %ecx
> >> andl $252645135, %ecx # imm = 0xF0F0F0F
> >> imull $16843009, %ecx, %eax # imm = 0x1010101
> >> shrl $24, %eax
> >> andl $1, %eax
> >> popq %rbp
> >> .cfi_def_cfa %rsp, 8
> >> retq
> >>
> >> JFTR: this is the same unoptimised code as shipped in the builtins
> library!
> >>
> >> Stefan
> >>
> >> > On Sun, Sep 6, 2020 at 1:32 PM Stefan Kanthak <
> stefan.kanthak at nexgo.de>
> >> > wrote:
> >> >
> >> >> "Craig Topper" <craig.topper at gmail.com> wrote;
> >> >>
> >> >>
> >> >>
> >> >> > Clang never generates calls to ___paritysi2, ___paritydi2,
> ___cmpdi2,
> >> or
> >> >>
> >> >> > ___ucmpdi2 on X86 so its not clear the performance of this matters
> at
> >> >> all.
> >> >>
> >> >>
> >> >>
> >> >> So you can safely remove them for X86 and all the other targets where
> >> such
> >> >>
> >> >> unoptimized code is never called!
> >> >>
> >> >> But fix these routines for targets where they are called.
> >> >>
> >> >>
> >> >>
> >> >> The statement does NOT make any exceptions, and it does not say
> >> >>
> >> >> | ships unoptimized routines the compiler never calls
> >> >>
> >> >> but
> >> >>
> >> >> | optimized target-independent implementations
> >> >>
> >> >>
> >> >>
> >> >> Stefan
> >> >>
> >> >>
> >> >>
> >> >> BTW: do builtins like __builtin_*parity* exist?
> >> >>
> >> >> If yes: do they generate the same bad code?
> >> >>
> >> >>
> >> >>
> >> >> > On Sun, Sep 6, 2020 at 12:31 PM Stefan Kanthak via cfe-dev <
> >> >>
> >> >> > cfe-dev at lists.llvm.org> wrote:
> >> >>
> >> >> >
> >> >>
> >> >> >> <https://compiler-rt.llvm.org/index.html> boasts:
> >> >>
> >> >> >>
> >> >>
> >> >> >> | The builtins library provides optimized implementations of this
> >> >>
> >> >> >> | and other low-level routines, either in target-independent C
> form,
> >> >>
> >> >> >> | or as a heavily-optimized assembly.
> >> >>
> >> >> >>
> >> >>
> >> >> >> Really?
> >> >>
> >> >> >>
> >> >>
> >> >> >> Left: inperformant code shipped in # Right: slightly improved
> >> code,
> >> >>
> >> >> >> clang_rt.builtins-* # which the optimiser
> >> >> REALLY
> >> >>
> >> >> >> # should have
> generated
> >> >>
> >> >> >>
> >> >>
> >> >> >> ___cmpdi2:
> >> >>
> >> >> >> mov ecx, [esp+16] # mov ecx,
> [esp+16]
> >> >>
> >> >> >> xor eax, eax # xor eax, eax
> >> >>
> >> >> >> cmp [esp+8], ecx # cmp ecx, [esp+8]
> >> >>
> >> >> >> jl @f # jg @f
> >> >>
> >> >> >> mov eax, 2 # mov eax, 2
> >> >>
> >> >> >> jg @f # jl @f
> >> >>
> >> >> >> mov ecx, [esp+4] #
> >> >>
> >> >> >> mov edx, [esp+12] # mov ecx,
> [esp+12]
> >> >>
> >> >> >> mov eax, 0 # xor eax, eax
> >> >>
> >> >> >> cmp ecx, edx # cmp ecx, [esp+4]
> >> >>
> >> >> >> jb @f # ja @f
> >> >>
> >> >> >> cmp edx, ecx #
> >> >>
> >> >> >> mov eax, 1 #
> >> >>
> >> >> >> adc eax, 0 # adc eax, 1
> >> >>
> >> >> >> @@: # @@:
> >> >>
> >> >> >> ret # ret
> >> >>
> >> >> >>
> >> >>
> >> >> >> # 3 instructions less, 10
> bytes
> >> >> saved
> >> >>
> >> >> >>
> >> >>
> >> >> >> ___ucmpdi2:
> >> >>
> >> >> >> mov ecx, [esp+16] # mov ecx,
> [esp+16]
> >> >>
> >> >> >> xor eax, eax # xor eax, eax
> >> >>
> >> >> >> cmp [esp+8], ecx # cmp ecx, [esp+8]
> >> >>
> >> >> >> jb @f # ja @f
> >> >>
> >> >> >> mov eax, 2 # mov eax, 2
> >> >>
> >> >> >> ja @f # jb @f
> >> >>
> >> >> >> mov ecx, [esp+4] #
> >> >>
> >> >> >> mov edx, [esp+12] # mov ecx,
> [esp+12]
> >> >>
> >> >> >> mov eax, 0 # xor eax, eax
> >> >>
> >> >> >> cmp ecx, edx # cmp ecx, [esp+4]
> >> >>
> >> >> >> jb @f # ja @f
> >> >>
> >> >> >> cmp edx, ecx #
> >> >>
> >> >> >> mov eax, 1 #
> >> >>
> >> >> >> adc eax, 0 # adc eax, 1
> >> >>
> >> >> >> @@: # @@:
> >> >>
> >> >> >> ret # ret
> >> >>
> >> >> >>
> >> >>
> >> >> >> # 3 instructions less, 10
> bytes
> >> >> saved
> >> >>
> >> >> >>
> >> >>
> >> >> >>
> >> >>
> >> >> >> Now properly written code, of course branch-free, faster and
> shorter:
> >> >>
> >> >> >>
> >> >>
> >> >> >> # Copyright (C) 2004-2020, Stefan Kanthak <
> stefan.kanthak at nexgo.de>
> >> >>
> >> >> >>
> >> >>
> >> >> >> ___cmpdi2:
> >> >>
> >> >> >> mov ecx, [esp+4]
> >> >>
> >> >> >> mov edx, [esp+12]
> >> >>
> >> >> >> cmp ecx, edx
> >> >>
> >> >> >> mov eax, [esp+8]
> >> >>
> >> >> >> sbb eax, [esp+16]
> >> >>
> >> >> >> setl ah
> >> >>
> >> >> >> cmp edx, ecx
> >> >>
> >> >> >> mov edx, [esp+16]
> >> >>
> >> >> >> sbb edx, [esp+8]
> >> >>
> >> >> >> setl al
> >> >>
> >> >> >> sub al, ah
> >> >>
> >> >> >> movsx eax, al
> >> >>
> >> >> >> inc eax
> >> >>
> >> >> >> ret
> >> >>
> >> >> >>
> >> >>
> >> >> >> ___ucmpdi2:
> >> >>
> >> >> >> mov ecx, [esp+4]
> >> >>
> >> >> >> mov edx, [esp+12]
> >> >>
> >> >> >> cmp ecx, edx
> >> >>
> >> >> >> mov eax, [esp+8]
> >> >>
> >> >> >> sbb eax, [esp+16]
> >> >>
> >> >> >> sbb eax, eax
> >> >>
> >> >> >> cmp edx, ecx
> >> >>
> >> >> >> mov edx, [esp+16]
> >> >>
> >> >> >> sbb edx, [esp+8]
> >> >>
> >> >> >> adc eax, 1
> >> >>
> >> >> >> ret
> >> >>
> >> >> >>
> >> >>
> >> >> >>
> >> >>
> >> >> >> AGAIN:
> >> >>
> >> >> >> Remove every occurance of the word "optimized" on the above web
> page.
> >> >>
> >> >> >>
> >> >>
> >> >> >> 'nuff said
> >> >>
> >> >> >> Stefan
> >> >>
> >> >> >> _______________________________________________
> >> >>
> >> >> >> cfe-dev mailing list
> >> >>
> >> >> >> cfe-dev at lists.llvm.org
> >> >>
> >> >> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >> >>
> >> >> >>
> >> >>
> >> >>
> >> >
> >> > --
> >> > ~Craig
> >> >
> >>
>
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