https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118678
Bug ID: 118678
Summary: Dubious optimization when compiling with -fpie -Os
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: joshudson at gmail dot com
Target Milestone: ---
The following code fragment demonstrates the problem:
extern int execve2(const char *name, const char**args, const char **environ);
int function()
{
const char *args[] = { "/bin/echo", "one", "two", "three", "four",
"five", "six", "seven", "eight", "nine", 0 };
return -execve2(args[0], args, 0);
}
When compiling with -S -Os it finds and optimization with the weight of the
code being eight bytes per array member:
leaq .LC11(%rip), %rsi
movl $22, %ecx
xorl %edx, %edx
leaq 8(%rsp), %rdi
rep movsl
It could have been better by saying
leaq .LC11(%rip), %rsi
movl $11, %ecx
xorl %edx, %edx
leaq 8(%rsp), %rdi
rep movsq
However the bug that surprised me is it continues to generate this movs
instruction when compiling with -S -Os -fpie ; the optimization is no longer
nice because this now requires *eleven* relocations with the weight of the code
being 16 bytes per array member; the simple solution generated by -S -O3
-mno-sse only uses 12 bytes per array member.
I don't really know how the guts of the optimizer work; but I haven't been able
to get it to generate this anywhere other than an array of constants, and it
often misses it; so I'm not too sure how hard this is to fix.
