Re: [Valgrind-users] Cubic root of zero gives wrong result with clang 64-bit

[Paul F. <pj...@wa...>]

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.


A+

Paul