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[Tinycc-devel] x64 -- bug with clobbered rbx register?

From: Bojan Nikolic
Subject: [Tinycc-devel] x64 -- bug with clobbered rbx register?
Date: Tue, 27 Mar 2012 22:28:53 +0100
User-agent: Gnus/5.110018 (No Gnus v0.18) Emacs/24.0.94 (gnu/linux)

Dear All,

I'm running into quite a basic problem with the x64 target on
Linux. After a while I tracked it down to an apparent clobbering of the
rbx register.

This snipped of code:

void f(double *a,  double *b,  void  *c)

generates following object code:

   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 81 ec 20 00 00 00    sub    $0x20,%rsp
   b:   48 89 7d f8             mov    %rdi,-0x8(%rbp)
   f:   48 89 75 f0             mov    %rsi,-0x10(%rbp)
  13:   48 89 55 e8             mov    %rdx,-0x18(%rbp)
  17:   48 8b 45 f8             mov    -0x8(%rbp),%rax
  1b:   48 89 45 e0             mov    %rax,-0x20(%rbp)
  1f:   b8 03 00 00 00          mov    $0x3,%eax
  24:   48 89 c7                mov    %rax,%rdi
  27:   b8 00 00 00 00          mov    $0x0,%eax
  2c:   e8 fc ff ff ff          callq  2d <f+0x2d>
  31:   f2 0f 2a c0             cvtsi2sd %eax,%xmm0
  35:   8b 45 e0                mov    -0x20(%rbp),%eax
  38:   f3 0f 7e c8             movq   %xmm0,%xmm1
  3c:   48 8b 5d e0             mov    -0x20(%rbp),%rbx
  40:   f3 0f 7e 03             movq   (%rbx),%xmm0
  44:   f2 0f 59 c1             mulsd  %xmm1,%xmm0
  48:   c9                      leaveq 
  49:   c3                      retq   

As you can see the rbx register gets clobbered by the load at address
0x3c. I'm not at all an expert on ABIs but I think the rbx needs to be
preserved across function calls and this is causing problems further
downstream in my code.

Any chance this could be easily addressed? If the rbx could be excluded
from registers used for general operations within functions that would
probably be sufficient for my purposes...


(I am  on git revision ID 83d57c06f406bbb7198b1683fc522f23251b5f57)

Bojan Nikolic          ||          http://www.bnikolic.co.uk

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