[llvm-dev] LLVM 10 is miscompiling this code on x86_64 with an invalid shift count for `shrd`

[Roman Lebedev via llvm-dev]

On Tue, Jan 26, 2021 at 9:28 PM Riyaz Puthiyapurayil via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> It appears that LLVM 10 is miscompiling the following test on x86_64 but the trunk version seems to work fine. Before I spend any time debugging this, it would be helpful if someone can point me to a bug fix if there was one.
>
>
>
> #include <stdio.h>
>
> #include <stdint.h>
>
>
>
> __attribute__ ((noinline)) uint64_t  foo4(unsigned* arr)
>
> {
>
>     unsigned chunk = 34 >> 5;
>
>     unsigned bitIndex = 34 - chunk * 32;
>
>
>
>     arr = arr + chunk;
>
>     __int128_t* arr1 = (__int128_t*) arr;
I'm pretty sure this is UB, because the alignment of __int128_t is
bigger than that of unsigned.

>
>     __int128_t v1 = *arr1;
>
>     v1 = v1 >> bitIndex;
>
>
>
>     return (uint64_t) v1;
>
> }
>
>
>
> __attribute__ ((noinline)) uint64_t  foo3(unsigned* arr, unsigned index)
>
> {
>
>     unsigned chunk = index >> 5;
>
>     unsigned bitIndex = index - chunk * 32;
>
>
>
>     arr = arr + chunk;
>
>     __int128_t* arr1 = (__int128_t*) arr;
>
>     __int128_t v1 = *arr1;
>
>     v1 = v1 >> bitIndex;
>
>
>
>     return (uint64_t) v1;
>
> }
>
>
>
>
>
> int main() {
>
>
>
>     unsigned arr[5];
>
>
>
>     arr[0]= 0x6f7cfd6f;
>
>     arr[1]= 0xd96c9806;
>
>     arr[2]= 0x89420144;
>
>     arr[3]= 0x8a20f548;
>
>
>
>     printf("foo3 %lx\n",foo3(arr,34) & 0x3ffffffff); // prints 222508051 instead of 1365b2601
>
>     printf("foo4 %lx\n",foo4(arr) & 0x3ffffffff); // prints 1365b2601 as expected
>
> }
>
>
>
> If you look at the assembly, there is an ‘and cl, 31’ missing in foo3 which is needed to make sure that the shift count is in the range [0...31]. This is missing in the generated code. Note that if the count in CL register is greater than 31, x86_64 manual states that the behavior of shrd is undefined.
>
>
>
> # clang 10
>
> foo3(unsigned int*, unsigned int):                             # @foo3(unsigned int*, unsigned int)
>
>         mov     ecx, esi
>
>         mov     edx, esi
>
>         shr     edx, 5
>
>         mov     rax, qword ptr [rdi + 4*rdx]
>
>         mov     rdx, qword ptr [rdi + 4*rdx + 8]
>
>         shrd    rax, rdx, cl
>
>         ret
>
>
>
> # Trunk
>
> foo3(unsigned int*, unsigned int):                             # @foo3(unsigned int*, unsigned int)
>
>         mov     ecx, esi
>
>         mov     edx, esi
>
>         shr     edx, 5
>
>         mov     rax, qword ptr [rdi + 4*rdx]
>
>         mov     rdx, qword ptr [rdi + 4*rdx + 8]
>
>         and     cl, 31
>
>         shrd    rax, rdx, cl
>
>         ret
>

Roman.

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