[llvm-dev] BUGS n code generated for target i386 compiling __bswapdi3, and for target x86-64 compiling __bswapsi2()
Craig Topper via llvm-dev
llvm-dev at lists.llvm.org
Sun Nov 25 12:26:33 PST 2018
I just compiled the two attached files in 32-bit mode and ran it.
It printed efcdab8967452301.
I verified via objdump that the my_bswap function contains the follow
assembly which I believe matches the assembly you linked to on godbolt.
_my_bswap:
1f70: 55 pushl %ebp
1f71: 89 e5 movl %esp, %ebp
1f73: 8b 55 08 movl 8(%ebp), %edx
1f76: 8b 45 0c movl 12(%ebp), %eax
1f79: 0f c8 bswapl %eax
1f7b: 0f ca bswapl %edx
1f7d: 5d popl %ebp
1f7e: c3 retl
~Craig
On Sun, Nov 25, 2018 at 11:39 AM Stefan Kanthak <stefan.kanthak at nexgo.de>
wrote:
> "Craig Topper" <craig.topper at gmail.com> wrote:
>
> > bswapdi2 for i386 is correct
>
> OUCH!
>
> > Bits 31:0 of the source are loaded into edx. Bits 63:32 are loaded into
> > eax. Those are each bswapped.
>
> This exchanges the high byte of each 32-bit PART with its low byte, but
> NOT the high byte of the whole 64-bit operand with its low byte!
>
> Please get a clue!
>
> > The ABI for the return is edx contains bits [63:32] and eax contains
> > [31:0]. This is opposite of how the register were loaded.
>
> My post is NOT about swapping EDX with EAX, but the bytes WITHIN both.
>
> With the 64-bit argument loaded into EDX:EAX, the instruction sequence
>
> bswap edx
> bswap eax
> xchg eax, edx
>
> is NOT equivalent to
>
> bswap rdi
>
> with the 64-bit argument loaded into RDI.
>
> Just run the following code on x86-64:
>
> mov rdi, 0123456789abcdefh ; pass (fake) argument in RDI
> ; split argument into high and low part
> mov rdx, rdi
> shr rdx, 32 ; high part in EDX
> mov eax, rdi ; low part in EAX
> ; perform __bswapdi2() as in 32-bit mode
> xchg eax, edx ; swap parts, argument now loaded
> ; like in 32-bit mode
> bswap edx
> bswap eax ; result like that in 32-bit mode
> ; load result into 64-bit register
> shl rdx, 32
> or rax, rdx
> ; perform _bswapdi2() in native 64-bit mode
> bswap rdi
> ; compare results
> xor rax, rdi
>
> not amused
> Stefan Kanthak
>
> > On Sun, Nov 25, 2018 at 10:36 AM Craig Topper <craig.topper at gmail.com>
> > wrote:
> >
> >> 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>
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> llvm-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>
> >>
>
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