https://llvm.org/bugs/show_bug.cgi?id=27589
Bug ID: 27589
Summary: large position independent x86 code
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
When compiling
extern int a;
int *f() {
return &a;
}
Clang generates the following for finding the current ip address:
f:
calll .L0$pb
.L0$pb:
popl %eax
.Ltmp0:
addl $_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %eax
movl a@GOT(%eax), %eax
ret
which is 19 bytes.
gcc produces
f:
call __x86.get_pc_thunk.ax
addl $_GLOBAL_OFFSET_TABLE_, %eax
movl a@GOT(%eax), %eax
ret
.section
.text.__x86.get_pc_thunk.ax,"axG",@progbits,__x86.get_pc_thunk.ax,comdat
.globl __x86.get_pc_thunk.ax
.hidden __x86.get_pc_thunk.ax
__x86.get_pc_thunk.ax:
movl (%esp), %eax
ret
While the total code is now 21 bytes, f is 17 bytes and __x86.get_pc_thunk.ax
will be merged by the linker.
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