https://bugs.kde.org/show_bug.cgi?id=359950
Bug ID: 359950
Summary: Wrong result comparing doubles on x87
Product: valgrind
Version: 3.10.0
Platform: Ubuntu Packages
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: general
Assignee: jsew...@acm.org
Reporter: wellnho...@aevum.de
This is on Ubuntu 15.04, x86 (32-bit). Using stock valgrind
1:3.10.1-1ubuntu3~15.04, gcc 4:4.9.2-2ubuntu2.
Test program:
#include <math.h>
#include <stdint.h>
#include <stdio.h>
int test(int64_t i, double d) {
double a = (double)i;
int comparison = (a == d || a < d || a > d);
printf("%lld %f\n", i, d);
return comparison;
}
int main() {
int64_t i = (INT64_C(1) << 60) - 1;
double d = pow(2.0, 60.0);
printf("%d\n", test(i, d));
return 0;
}
When compiled with -O2 and run under valgrind, the result is:
==3514== Memcheck, a memory error detector
==3514== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==3514== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==3514== Command: ./vg_bug
==3514==
1152921504606846975 1152921504606846976.000000
0
==3514==
==3514== HEAP SUMMARY:
==3514== in use at exit: 0 bytes in 0 blocks
==3514== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==3514==
==3514== All heap blocks were freed -- no leaks are possible
==3514==
==3514== For counts of detected and suppressed errors, rerun with: -v
==3514== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
All three comparisons returned false.
Dump of assembler code for function test:
0x08048480 <+0>: push %esi
0x08048481 <+1>: push %ebx
0x08048482 <+2>: sub $0x14,%esp
0x08048485 <+5>: mov 0x20(%esp),%ecx
0x08048489 <+9>: mov 0x24(%esp),%ebx
0x0804848d <+13>: fldl 0x28(%esp)
0x08048491 <+17>: mov %ecx,(%esp)
0x08048494 <+20>: mov %ebx,0x4(%esp)
0x08048498 <+24>: fildll (%esp)
0x0804849b <+27>: fucomi %st(1),%st
0x0804849d <+29>: jp 0x80484a6 <test+38>
0x0804849f <+31>: mov $0x1,%esi
0x080484a4 <+36>: je 0x80484b8 <test+56>
0x080484a6 <+38>: xor %eax,%eax
0x080484a8 <+40>: fucomip %st(1),%st
0x080484aa <+42>: setne %al
0x080484ad <+45>: mov %eax,%esi
0x080484af <+47>: jmp 0x80484ba <test+58>
0x080484b1 <+49>: lea 0x0(%esi,%eiz,1),%esi
0x080484b8 <+56>: fstp %st(0)
0x080484ba <+58>: sub $0x10,%esp
0x080484bd <+61>: fstpl (%esp)
0x080484c0 <+64>: push %ebx
0x080484c1 <+65>: push %ecx
0x080484c2 <+66>: push $0x8048560
0x080484c7 <+71>: push $0x1
0x080484c9 <+73>: call 0x8048330 <__printf_chk@plt>
0x080484ce <+78>: add $0x34,%esp
0x080484d1 <+81>: mov %esi,%eax
0x080484d3 <+83>: pop %ebx
0x080484d4 <+84>: pop %esi
0x080484d5 <+85>: ret
The strange thing is that this works as expected if the first printf statement
in function test is omitted. Without the first printf:
Dump of assembler code for function test:
0x08048460 <+0>: sub $0xc,%esp
0x08048463 <+3>: fldl 0x18(%esp)
0x08048467 <+7>: fildll 0x10(%esp)
0x0804846b <+11>: fstpl (%esp)
0x0804846e <+14>: fldl (%esp)
0x08048471 <+17>: fucomi %st(1),%st
0x08048473 <+19>: jp 0x804847c <test+28>
0x08048475 <+21>: mov $0x1,%eax
0x0804847a <+26>: je 0x8048490 <test+48>
0x0804847c <+28>: xor %eax,%eax
0x0804847e <+30>: fucomip %st(1),%st
0x08048480 <+32>: fstp %st(0)
0x08048482 <+34>: setne %al
0x08048485 <+37>: jmp 0x8048494 <test+52>
0x08048487 <+39>: mov %esi,%esi
0x08048489 <+41>: lea 0x0(%edi,%eiz,1),%edi
0x08048490 <+48>: fstp %st(0)
0x08048492 <+50>: fstp %st(0)
0x08048494 <+52>: add $0xc,%esp
0x08048497 <+55>: ret
Reproducible: Always
Steps to Reproduce:
1. gcc -Wall -O2 vg_bug.c -o vg_bug
2. valgrind ./vg_bug
Actual Results:
==3617== Memcheck, a memory error detector
==3617== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==3617== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==3617== Command: ./vg_bug
==3617==
1152921504606846975 1152921504606846976.000000
0
==3617==
==3617== HEAP SUMMARY:
==3617== in use at exit: 0 bytes in 0 blocks
==3617== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==3617==
==3617== All heap blocks were freed -- no leaks are possible
==3617==
==3617== For counts of detected and suppressed errors, rerun with: -v
==3617== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Expected Results:
==3617== Memcheck, a memory error detector
==3617== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==3617== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==3617== Command: ./vg_bug
==3617==
1152921504606846975 1152921504606846976.000000
1
==3617==
==3617== HEAP SUMMARY:
==3617== in use at exit: 0 bytes in 0 blocks
==3617== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==3617==
==3617== All heap blocks were freed -- no leaks are possible
==3617==
==3617== For counts of detected and suppressed errors, rerun with: -v
==3617== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
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