https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65247
Bug ID: 65247
Summary: [missed optimisation] Reading a callee-saved register
writes to the stack
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: adam at consulting dot net.nz
There is no way to inform the compiler one is reading a callee-saved register
without the compiler also writing the register to the stack:
read_callee_saved_register.c:
#include <stdint.h>
uint64_t read_rbx(void) {
register uint64_t rbx asm ("rbx");
uint64_t value;
asm ("mov %[rbx], %[value]" : [value] "=r" (value) : [rbx] "r" (rbx));
return value;
}
int main(void) {
return 0;
}
The code above explicitly tells the complier that local register variable rbx
is only being read. Here is the output of gcc and clang respectively:
$ gcc-snapshot.sh -O3 read_callee_saved_register.c && objdump -d -m i386:x86-64
a.out|less
00000000004004c0 <read_rbx>:
4004c0: 48 89 d8 mov %rbx,%rax
4004c3: 53 push %rbx
4004c4: 5b pop %rbx
4004c5: c3 retq
4004c6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
4004cd: 00 00 00
$ clang-3.7 -O3 read_callee_saved_register.c && objdump -d -m i386:x86-64
a.out|less
00000000004004e0 <read_rbx>:
4004e0: 53 push %rbx
4004e1: 48 89 d8 mov %rbx,%rax
4004e4: 5b pop %rbx
4004e5: c3 retq
4004e6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
4004ed: 00 00 00
Neither gcc nor clang should be pushing and popping rbx.
Note: gcc already trusts that rbx is merely being read since the assembly is
inserted before rbx is pushed and popped off the stack.
