[Bug rtl-optimization/65247] New: [missed optimisation] Reading a callee-saved register writes to the stack

[Bug rtl-optimization/65247] New: [missed optimisation] Reading a callee-saved register writes to the stack

[adam at consulting dot net.nz]

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.

[Bug rtl-optimization/65247] [missed optimisation] Reading a callee-saved register writes to the stack

[adam at consulting dot net.nz]

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

--- Comment #1 from Adam Warner <adam at consulting dot net.nz> ---
CLANG bug report:
<http://llvm.org/bugs/show_bug.cgi?id=22735>

[Bug rtl-optimization/65247] [missed optimisation] Reading a callee-saved register writes to the stack

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization, ra
             Target|                            |x86_64-*-*, i?86-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-03-02
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, if you want to read %rbx as it is at the point of the asm (you don't
initialize "rbx") then simply read from it without any input or input
constraint.
Or do not use an asm at tall but just 'return rbx;'?

As of why we still push/pop %rbx, no idea.  That is, confirmed with

#include <stdint.h>

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