[llvm-dev] BUGS n code generated for target i386 compiling __bswapdi3, and for target x86-64 compiling __bswapsi2()

[Craig Topper via llvm-dev]

bswapsi2 on the x86-64 isn't using the bswap instruction because "unsigned
long" is 64-bits on x86-64 linux. But its 32-bits on x86-64 msvc.

Not sure about the bswapdi2 i386 case.


~Craig


On Sun, Nov 25, 2018 at 8:03 AM Stefan Kanthak via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi @ll,
>
> targetting i386, LLVM/clang generates wrong code for the following
> functions:
>
> unsigned long __bswapsi2 (unsigned long ul)
> {
>     return (((ul) & 0xff000000ul) >> 3 * 8)
>          | (((ul) & 0x00ff0000ul) >>     8)
>          | (((ul) & 0x0000ff00ul) <<     8)
>          | (((ul) & 0x000000fful) << 3 * 8);
> }
>
> unsigned long long __bswapdi2(unsigned long long ull)
> {
>     return ((ull & 0xff00000000000000ull) >> 7 * 8)
>          | ((ull & 0x00ff000000000000ull) >> 5 * 8)
>          | ((ull & 0x0000ff0000000000ull) >> 3 * 8)
>          | ((ull & 0x000000ff00000000ull) >>     8)
>          | ((ull & 0x00000000ff000000ull) <<     8)
>          | ((ull & 0x0000000000ff0000ull) << 3 * 8)
>          | ((ull & 0x000000000000ff00ull) << 5 * 8)
>          | ((ull & 0x00000000000000ffull) << 7 * 8);
> }
>
> You can find these sources in "compiler-rt/lib/builtins/bswapsi2.c"
> and "compiler-rt/lib/builtins/bswapdi2.c", for example!
>
>
> Compiled with "-O3 -target i386" this yields the following code
> (see <https://godbolt.org/z/F4UIl4>):
>
> __bswapsi2: # @__bswapsi2
>     push  ebp
>     mov   ebp, esp
>     mov   eax, dword ptr [ebp + 8]
>     bswap eax
>     pop   ebp
>     ret
>
> __bswapdi2: # @__bswapdi2
>     push  ebp
>     mov   ebp, esp
>     mov   edx, dword ptr [ebp + 8]
>     mov   eax, dword ptr [ebp + 12]
>     bswap eax
>     bswap edx
>     pop   ebp
>     ret
>
> __bswapsi2() is correct, but __bswapdi2() NOT: swapping just the
> halves of a "long long" is OBVIOUSLY WRONG!
>
> From the C source, the expected result for the input value
> 0x0123456789ABCDEF is 0xEFCDAB8967452301; the compiled code but
> produces 0x67452301EFCDAB89
>
>
> And compiled for x86-64 this yields the following code (see
> <https://godbolt.org/z/uM9nvN>):
>
> __bswapsi2: # @__bswapsi2
>     mov   eax, edi
>     shr   eax, 24
>     mov   rcx, rdi
>     shr   rcx, 8
>     and   ecx, 65280
>     or    rax, rcx
>     mov   rcx, rdi
>     shl   rcx, 8
>     and   ecx, 16711680
>     or    rax, rcx
>     and   rdi, 255
>     shl   rdi, 24
>     or    rax, rdi
>     ret
>
> __bswapdi2: # @__bswapdi2
>     bswap rdi
>     mov   rax, rdi
>     ret
>
> Both are correct, but __bswapsi2() should of course use BSWAP too!
>
>
> Stefan Kanthak
>
> PS: for comparision with another compiler, take a look at
>     <https://skanthak.homepage.t-online.de/msvc.html#example5>
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