http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53617
Bug #: 53617
Summary: Suboptimal PC-relative addressing code on x86
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bug...@aerifal.cx
Consider a function of the form:
int *foo()
{
static int x;
return &x;
}
When GCC compiles this, the result is similar to the following hand-written
asm:
call 1f
1: pop %ecx
add $_GLOBAL_OFFSET_TABLE_+[.-1b], %ecx
lea x@GOTOFF(%ecx),%eax
ret
i.e. it loads the address of the GOT in a register, then computes the
GOT-relative address of the variable. This makes sense when the GOT address
will be needed for more than one address lookup, but it's unnecessarily costly
in the case where it's only used for a single variable, in which case the
following code would be better:
call 1f
1: pop %eax
add $[x-1b], %eax
ret
The same principle applies to a great deal more code, and of course it works
when __i686.get_pc_thunk.cx or similar is being used, too. Is there any way GCC
could be enhanced to generate this superior code when possible?
