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 : valgrind-users@lists.sourceforge.net
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 (g...@github.com: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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