Re: Cubic root of zero gives wrong result with clang 64-bit
Paul Floyd <[email protected]>
| Newsgroups | gmane.comp.debugging.valgrind |
|---|---|
| Message-ID | <[email protected]> |
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 : [email protected] >>> >>> 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 ([email protected]: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 _______________________________________________ Valgrind-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/valgrind-users