https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71475
Bug ID: 71475
Summary: Optimization of copying into long double looses bytes
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ch3root at openwall dot com
Target Milestone: ---
It seems that copying of bytes into a long double with memcpy or in some other
way is sometimes optimized by converting these bytes into long double at
compile-time and the process looses two higher bytes -- see the output "after
memcpy" with -O3 (bytes are printed starting from the big end).
Source code:
----------------------------------------------------------------------
#include <string.h>
#include <stdio.h>
int main()
{
long double d0, d;
memset(&d0, 'A', sizeof d0);
d = d0;
printf("after memset: %Lf\n", d);
for (unsigned char *p = (unsigned char *)&d + sizeof d; p-- > (unsigned char
*)&d;)
printf("%02x ", *p);
printf("\n");
printf("\n");
memcpy(&d0, "AAAAAAAAAAAAAAAA", sizeof d0);
d = d0;
printf("after memcpy: %Lf\n", d);
for (unsigned char *p = (unsigned char *)&d + sizeof d; p-- > (unsigned char
*)&d;)
printf("%02x ", *p);
printf("\n");
}
----------------------------------------------------------------------
Results:
----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra test.c && ./a.out
after memset:
4355738269328914677589017258057892854796664468317418542308867623691847702374177028736570422722560.000000
00 00 00 00 00 00 41 41 41 41 41 41 41 41 41 41
after memcpy:
4355738269328914677589017258057892854796664468317418542308867623691847702374177028736570422722560.000000
00 00 00 00 00 00 41 41 41 41 41 41 41 41 41 41
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
after memset:
4355738269328914677589017258057892854796664468317418542308867623691847702374177028736570422722560.000000
00 00 00 00 00 40 41 41 41 41 41 41 41 41 41 41
after memcpy: 0.000000
00 00 00 00 00 40 00 00 41 41 41 41 41 41 41 41
----------------------------------------------------------------------
gcc version: gcc (GCC) 7.0.0 20160608 (experimental)
