[LLVMbugs] [Bug 22735] New: [missed optimisation] Reading a callee-saved register writes to the stack

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22735

            Bug ID: 22735
           Summary: [missed optimisation] Reading a callee-saved register
                    writes to the stack
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: adam at consulting.net.nz
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

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.

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