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Re: [Valgrind-users] Cubic root of zero gives wrong result with clang 64-bit
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From: Paul F. <pj...@wa...> - 2021-10-20 19:41:07
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On 20/10/2021 20:13, Paul Floyd wrote:
> On 10/20/21 16:16, Paul FLOYD wrote:
>
>>> Message du 20/10/21 17:14
>>>
>>> De : "Vladislav Yaglamunov"
>>>
>>> A : val...@li...
>>>
>>> Copie à :
>>>
>>> Objet : [Valgrind-users] Cubic root of zero gives wrong result with
>>> clang 64-bit
>>>
>>> I am using Valgrind 3.17.0 and noticed a strange behavior while
>>> running a code compiled with clang:
>>>
>> #include
>>
>>> int main() {
>>>
>>> long double x = std::cbrtl(0.0L);
>>>
>>> printf("%Lf", x);
>>>
>>> return 0;
>>>
>>> }
>>>
>> I just did a test with g++. I had to modify the code a bit to prevent
>> the compiler replacing the cbrtl() call with just a zero:
>>
>> #include <cmath>
>>
>> #include <
>>
>> int main() {
>>
>> volatile long double arg{0.0L};
>>
>> long double x = std::cbrtl(arg);
>>
>> printf("%Lf", x);
>>
>> return 0;
>>
>> }
>>
>> This seems to work OK, both default and -m32.
>>
>> The asm for that is
>>
>> 00000000004011a5 :
>>
>> 4011a5: 55 push %rbp
>>
>> 4011a6: 48 89 e5 mov %rsp,%rbp
>>
>> 4011a9: 48 83 ec 20 sub $0x20,%rsp
>>
>> 4011ad: d9 ee fldz
>>
>> 4011af: db 7d e0 fstpt -0x20(%rbp)
>>
>> 4011b2: db 6d e0 fldt -0x20(%rbp)
>>
>> 4011b5: 48 8d 64 24 f0 lea -0x10(%rsp),%rsp
>>
>> 4011ba: db 3c 24 fstpt (%rsp)
>>
>> 4011bd: e8 8e fe ff ff callq 401050
>>
>> 4011c2: 48 83 c4 10 add $0x10,%rsp
>>
>> 4011c6: db 7d f0 fstpt -0x10(%rbp)
>>
>> 4011c9: ff 75 f8 pushq -0x8(%rbp)
>>
>> 4011cc: ff 75 f0 pushq -0x10(%rbp)
>>
>> 4011cf: bf 04 20 40 00 mov $0x402004,%edi
>>
>> 4011d4: b8 00 00 00 00 mov $0x0,%eax
>>
>> 4011d9: e8 62 fe ff ff callq 401040
>>
>> 4011de: 48 83 c4 10 add $0x10,%rsp
>>
>> 4011e2: b8 00 00 00 00 mov $0x0,%eax
>>
>> 4011e7: c9 leaveq
>>
>> 4011e8: c3 retq
>>
> Well, it works with clang++:
>
> FreeBSD clang version 11.0.1 (gi...@gi...:llvm/llvm-project.git
> llvmorg-11.0.
>
> 1-0-g43ff75f2c3fe)
>
> Target: x86_64-unknown-freebsd13.0
>
> I also tried clang++ 8, 9 and 12.
>
> The asm for clang++12 for my modified code is
>
> 0000000000201920 <main>:
>
> 201920: 55 push %rbp
>
> 201921: 48 89 e5 mov %rsp,%rbp
>
> 201924: 48 83 ec 40 sub $0x40,%rsp
>
> 201928: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
>
> 20192f: d9 ee fldz
>
> 201931: db 7d e0 fstpt -0x20(%rbp)
>
> 201934: db 6d e0 fldt -0x20(%rbp)
> 201937: 48 89 e0 mov %rsp,%rax
> 20193a: db 38 fstpt (%rax)
> 20193c: e8 bf 00 00 00 call 201a00 <cbrtl@plt>
> 201941: db 7d d0 fstpt -0x30(%rbp)
> 201944: db 6d d0 fldt -0x30(%rbp)
> 201947: 48 89 e0 mov %rsp,%rax
> 20194a: db 38 fstpt (%rax)
> 20194c: bf c9 05 20 00 mov $0x2005c9,%edi
> 201951: 31 c0 xor %eax,%eax
> 201953: e8 b8 00 00 00 call 201a10 <printf@plt>
> 201958: 31 c0 xor %eax,%eax
> 20195a: 48 83 c4 40 add $0x40,%rsp
> 20195e: 5d pop %rbp
> 20195f: c3 ret
>
> Can you post the asm that you are getting ? I'll see if I can try
> clang on Fedora 34.
>
Third time lucky. Reproduced on Fedora 34 with clang++
clang version 12.0.1 (Fedora 12.0.1-1.fc34)
0000000000401140 <main>:
401140: 55 push %rbp
401141: 48 89 e5 mov %rsp,%rbp
401144: 48 83 ec 40 sub $0x40,%rsp
401148: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
40114f: d9 ee fldz
401151: db 7d e0 fstpt -0x20(%rbp)
401154: db 6d e0 fldt -0x20(%rbp)
401157: 48 89 e0 mov %rsp,%rax
40115a: db 38 fstpt (%rax)
40115c: e8 df fe ff ff callq 401040 <cbrtl@plt>
401161: db 7d d0 fstpt -0x30(%rbp)
401164: db 6d d0 fldt -0x30(%rbp)
401167: 48 89 e0 mov %rsp,%rax
40116a: db 38 fstpt (%rax)
40116c: bf 10 20 40 00 mov $0x402010,%edi
401171: 31 c0 xor %eax,%eax
401173: e8 b8 fe ff ff callq 401030 <printf@plt>
401178: 31 c0 xor %eax,%eax
40117a: 48 83 c4 40 add $0x40,%rsp
40117e: 5d pop %rbp
40117f: c3 retq
That looks very much like what I saw on FreeBSD.
Unless someone else has an Idea this is going to need some debugging
inside Valgrind.
A+
Paul
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