[cfe-dev] [llvm-dev] A pattern for portable __builtin_add_overflow()

[Sanjoy Das via cfe-dev]

You can grep for uses of the sadd_with_overflow intrinsic in the LLVM
code base to find other patterns that we may recognize.  If there are
no portable ones you might have to add your own pattern.

-- Sanjoy

On Thu, Nov 22, 2018 at 12:08 PM Paweł Bylica <chfast at gmail.com> wrote:
>
> Thanks Sanjoy,
>
> It looks it works only up to 32-bit integers (http://llvm.org/doxygen/InstCombineCompares_8cpp_source.html#l01255).
> I will need it also for 64-bit integers.
>
> Even if the pattern match would handle the 64-bit case I would need to use 128-bit types what is not portable still.
>
> // P.
>
> On Thu, Nov 22, 2018 at 6:54 PM Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
>>
>> Going by InstCombiner::foldICmpWithConstant,
>> https://gcc.godbolt.org/z/To_qmm should work.
>>
>> -- Sanjoy
>> On Tue, Nov 20, 2018 at 2:46 AM Paweł Bylica via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi LLVM, clang,
>> >
>> > I'm trying to write a portable version of __builtin_add_overflow() it a way that the compiler would
>> > recognize the pattern and use the add_overflow intrinsic / the best possible machine instruction.
>> >
>> > Here are docs about these builtins: https://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins.
>> >
>> > With unsigned types this is easy:
>> >
>> > int uaddo_native(unsigned a, unsigned b, unsigned* s)
>> > {
>> > return __builtin_add_overflow(a, b, s);
>> > }
>> >
>> > int uaddo_portable(unsigned a, unsigned b, unsigned* s)
>> > {
>> > *s = a + b;
>> > return *s < a;
>> > }
>> >
>> > We get exactly the same assembly:
>> > uaddo_native: # @uaddo_native
>> > xor eax, eax
>> > add edi, esi
>> > setb al
>> > mov dword ptr [rdx], edi
>> > ret
>> > uaddo_portable: # @uaddo_portable
>> > xor eax, eax
>> > add edi, esi
>> > setb al
>> > mov dword ptr [rdx], edi
>> > ret
>> >
>> > But with signed types it is not so easy. I tried 2 versions, but the result is quite far away from the optimal assembly.
>> >
>> >
>> > int saddo_native(int a, int b, int* s)
>> > {
>> > return __builtin_add_overflow(a, b, s);
>> > }
>> >
>> > int saddo_portable(int a, int b, int* s)
>> > {
>> > *s = (unsigned)a + (unsigned)b;
>> > return (a > 0) ? *s <= b : *s > b;
>> > }
>> >
>> > int saddo_portable2(int a, int b, int* s)
>> > {
>> > *s = (unsigned)a + (unsigned)b;
>> > int cond = a > 0;
>> > int check = *s > b;
>> > return (cond & !check) | (!cond & check);
>> > }
>> >
>> > Assembly:
>> >
>> > saddo_native: # @saddo_native
>> > xor eax, eax
>> > add edi, esi
>> > seto al
>> > mov dword ptr [rdx], edi
>> > ret
>> > saddo_portable: # @saddo_portable
>> > lea eax, [rsi + rdi]
>> > mov dword ptr [rdx], eax
>> > cmp eax, esi
>> > setle al
>> > setg cl
>> > test edi, edi
>> > jg .LBB3_2
>> > mov eax, ecx
>> > .LBB3_2:
>> > movzx eax, al
>> > ret
>> > saddo_portable2: # @saddo_portable2
>> > lea eax, [rsi + rdi]
>> > mov dword ptr [rdx], eax
>> > test edi, edi
>> > setg cl
>> > cmp eax, esi
>> > setg al
>> > xor al, cl
>> > movzx eax, al
>> > ret
>> >
>> >
>> > Do you know the trick to force the compiler to use the seto instruction?
>> >
>> > I also noticed that the transformation for uaddo_portable happens in CodeGen, not in IR.
>> >
>> > Bests,
>> > Paweł
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev