[Bug c/33527] New: GCC doesn't protect %edi when using inline assembly

[Bug c/33527] New: GCC doesn't protect %edi when using inline assembly

[meng dot yan at gmail dot com]

OS: Ubuntu 7.04
GCC version: 4.2.1

I try to play the inline assembly function. Here I use "rep movsb" to copy a
string.

// Code begin
char input[] = {"GCC Version Number"};
char output[30];
int  length = 28;

asm (
  "cld\n\t"
  "rep movsb"
  :
  : "c"(length), "S"(input), "D"(output));

printf("%s\n", output);
// Code end

  While compiled without "-O", everything is okay. However, if compiled with
"-O", the compiler will generate incorrect code as if the %edi never changes
during the assembly code execution.

Compile with "-S" will see the corresponding generated assembly codes are:

// code begin
# APP
  cld
  rep movsb
# NO_APP
  movl %edi, -4(%esp)  # BAD CODE HERE
  movl $.LC1, (%esp)
  call printf 
// code end

The problem is, movsb changes the value of %esi and %edi, and I told the
compiler about this in the "input" section in the asm directive. The compiler
didn't notice this and believe the value of %edi unchanged. 

I think this is a bug.


-- 
           Summary: GCC doesn't protect %edi when using inline assembly
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: meng dot yan at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33527

[Bug c/33527] GCC doesn't protect %edi when using inline assembly

[ebotcazou at gcc dot gnu dot org]

------- Comment #1 from ebotcazou at gcc dot gnu dot org  2007-09-22 07:27 -------
> The problem is, movsb changes the value of %esi and %edi, and I told the
> compiler about this in the "input" section in the asm directive.

No, you didn't, you only told that %esi and %edi should be loaded on entry.

See section 5.35 of the manual, especially the paragraph starting with "Some
instructions clobber specific hard registers".  Note that the provision "You
may not write a clobber description in a way that overlaps with an input or
output operand" will apply in this case, so 'volatile' will be required.


-- 

ebotcazou at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33527

[Bug c/33527] GCC doesn't protect %edi when using inline assembly

[ebotcazou at libertysurf dot fr]

------- Comment #2 from ebotcazou at libertysurf dot fr  2007-09-22 10:22 -------
Subject: Re:  GCC doesn't protect %edi when using inline assembly

> I've tried 'volatile', but it doesn't work.

#include <stdio.h>

int main(void)
{
char input[] = {"GCC Version Number"};
char output[30],*dummy;
int  length = 28;

asm volatile (
  "cld\n\t"
  "rep movsb"
  : "=S"(dummy),"=D"(dummy)
  : "c"(length), "0"(input), "1"(output));

printf("%s\n", output);

return 0;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33527