[llvm-dev] LLVM 10 is miscompiling this code on x86_64 with an invalid shift count for `shrd`
Roman Lebedev via llvm-dev
llvm-dev at lists.llvm.org
Tue Jan 26 11:31:23 PST 2021
https://godbolt.org/z/5bcsj9
example.c:43:37: runtime error: subtraction of unsigned offset from
0x000000000022 overflowed to 0x7fffd01217c8
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior example.c:43:37 in
example.c:19:37: runtime error: applying non-zero offset
140576899311424 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior example.c:19:37 in
foo3 19bdf3f5b
foo4 19bdf3f5b
On Tue, Jan 26, 2021 at 10:29 PM Riyaz Puthiyapurayil
<Riyaz.Puthiyapurayil at synopsys.com> wrote:
>
> Apologies, outlook messed up the formatting in the previous email. Here is it again.
>
>
>
> Do you see any UB in the following modified test? Here also, I see a difference between foo3 and foo4. Again, trunk and gcc match. But not Clang-10.
>
>
>
> #include <stdio.h>
>
> #include <stdint.h>
>
> #include <string.h>
>
>
>
> __attribute__ ((noinline)) uint64_t foo4(__int128_t* arr1)
>
> {
>
> unsigned chunk = 34 >> 5;
>
> unsigned bitIndex = 34 - chunk * 32;
>
>
>
> unsigned* arr = (unsigned *)arr + chunk;
>
> __int128_t v1 = *arr1;
>
> v1 = v1 >> bitIndex;
>
>
>
> return (uint64_t) v1;
>
> }
>
>
>
> __attribute__ ((noinline)) uint64_t foo3(__int128_t* arr1, unsigned index)
>
> {
>
> unsigned chunk = index >> 5;
>
> unsigned bitIndex = index - chunk * 32;
>
>
>
> unsigned *arr = (unsigned *)arr + chunk;
>
> __int128_t v1 = *arr1;
>
> v1 = v1 >> bitIndex;
>
>
>
> return (uint64_t) v1;
>
> }
>
>
>
> int main() {
>
>
>
> unsigned arr[4];
>
>
>
> arr[0]= 0x6f7cfd6f;
>
> arr[1]= 0xd96c9806;
>
> arr[2]= 0x89420144;
>
> arr[3]= 0x8a20f548;
>
>
>
> __int128_t arr1;
>
>
>
> memcpy(&arr1, arr, sizeof(arr1));
>
>
>
> printf("foo3 %lx\n",foo3(&arr1,34) & 0x3ffffffff);
>
> printf("foo4 %lx\n",foo4(&arr1) & 0x3ffffffff);
>
> }
>
>
>
>
>
>
>
> From: Riyaz Puthiyapurayil
> Sent: Tuesday, January 26, 2021 11:25 AM
> To: 'Roman Lebedev' <lebedev.ri at gmail.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: RE: [llvm-dev] LLVM 10 is miscompiling this code on x86_64 with an invalid shift count for `shrd`
>
>
>
> Do you see any UB in the following modified test? Here also, I see a difference between foo3 and foo4. Again, trunk and gcc match. But not Clang-10
>
>
>
> #include <stdio.h>
>
> #include <stdint.h>
>
> #include <string.h>
>
>
>
> __attribute__ ((noinline)) uint64_t foo4(__int128_t* arr1)
>
> {
>
> unsigned chunk = 34 >> 5;
>
> unsigned bitIndex = 34 - chunk * 32;
>
>
>
> unsigned* arr = (unsigned *)arr + chunk;
>
> __int128_t v1 = *arr1;
>
> v1 = v1 >> bitIndex;
>
>
>
> return (uint64_t) v1;
>
> }
>
>
>
> __attribute__ ((noinline)) uint64_t foo3(__int128_t* arr1, unsigned index)
>
> {
>
> unsigned chunk = index >> 5;
>
> unsigned bitIndex = index - chunk * 32;
>
>
>
> unsigned *arr = (unsigned *)arr + chunk;
>
> __int128_t v1 = *arr1;
>
> v1 = v1 >> bitIndex;
>
>
>
> return (uint64_t) v1;
>
> }
>
>
>
> int main() {
>
>
>
> unsigned arr[4];
>
>
>
> arr[0]= 0x6f7cfd6f;
>
> arr[1]= 0xd96c9806;
>
> arr[2]= 0x89420144;
>
> arr[3]= 0x8a20f548;
>
>
>
> __int128_t arr1;
>
>
>
> memcpy(&arr1, arr, sizeof(arr1));
>
>
>
> printf("foo3 %lx\n",foo3(&arr1,34) & 0x3ffffffff); // prints 1365b2601 (clang-10) 19bdf3f5b (gcc, clang-12)
>
> printf("foo4 %lx\n",foo4(&arr1) & 0x3ffffffff); // prints 19bdf3f5b
>
> }
>
>
>
> -----Original Message-----
> From: Riyaz Puthiyapurayil
> Sent: Tuesday, January 26, 2021 11:10 AM
> To: 'Roman Lebedev' <lebedev.ri at gmail.com>
> Subject: RE: [llvm-dev] LLVM 10 is miscompiling this code on x86_64 with an invalid shift count for `shrd`
>
>
>
> You may be right about that the cast from unsigned* to __int128_t* being UB. But I think that has nothing to do with the bug. That code can be fixed to use a __int128_t array and change all the casts from __int128_t* to unsigned* (instead of the other way around) and we would still have the issue.
>
>
>
> -----Original Message-----
>
> From: Roman Lebedev <lebedev.ri at gmail.com>
>
> Sent: Tuesday, January 26, 2021 10:34 AM
>
> To: Riyaz Puthiyapurayil <riyaz at synopsys.com>
>
> Cc: llvm-dev at lists.llvm.org
>
> Subject: Re: [llvm-dev] LLVM 10 is miscompiling this code on x86_64 with an invalid shift count for `shrd`
>
>
>
> 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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>
> > llvm-dev at lists.llvm.org
>
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>
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>
> > rh1gbrtqvDoSi6cMuSLZaXIPgtijPW55JJ_8$
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