[missed optimisation] Reading a callee-saved register writes to the stack

[llvmbot]

Bugzilla Link	22735
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid

Extended Description

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

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 : 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 : 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.

[llvmbot]

The unnecessary pushing and popping has generally been fixed in clang 3.7 and above (thanks!) But the simple function read_rbx() may expose a long-standing bug in clang:

include

uint64_t read_rbx(void) { register uint64_t rbx asm ("rbx"); return rbx; }

uint64_t read_rbx2(void) { register uint64_t rbx asm ("rbx"); uint64_t val; asm ("mov %1, %0" : "=r" (val) : "r" (rbx)); return val; }

int main(void) { return 0; }

$ clang-3.7 -O3 read_rbx.c && objdump -d -m i386:x86-64:intel a.out|less

00000000004004c0 : 4004c0: c3 ret
4004c1: 66 66 66 66 66 66 2e data16 data16 data16 data16 data16 nop WORD PTR cs:[rax+rax*1+0x0] 4004c8: 0f 1f 84 00 00 00 00 4004cf: 00

00000000004004d0 : 4004d0: 48 89 d8 mov rax,rbx 4004d3: c3 ret
4004d4: 66 66 66 2e 0f 1f 84 data16 data16 nop WORD PTR cs:[rax+rax*1+0x0] 4004db: 00 00 00 00 00

According to GCC developer Richard Biener https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65247#c2 read_rbx() is intended to return the value of RBX in RAX.

[llvmbot]

GCC bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65247