FTBFS on i386 in quantal

Reported by Michael Terry on 2012-07-27
This bug affects 1 person
Affects Status Importance Assigned to Milestone
libsdl-perl (Ubuntu)
sdlgfx (Debian)
Fix Released
sdlgfx (Ubuntu)

Bug Description

libsdl-perl fails on i386 in quantal due to a test failure:

# Failed test 'MMX_detect (MMX is same as at start)'
# at t/gfx_imagefilter.t line 53.
# got: '1'
# expected: '0'
# Looks like you failed 1 test of 8.
t/gfx_imagefilter.t .............
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/8 subtests

Which is really a failure in sdlgfx's SDL_imageFilterMMXdetect call, which seems to be bogus? It uses a function _cpuFlags() written in assembly that does not seem to do what it should. Specifically, multiple calls to SDL::GFX::ImageFilter::MMX_detect() seem to alternate between "on" or "off" by itself.

Its code is:

 asm volatile ("pusha \n\t" "mov %1, %%eax \n\t" /* request feature flag */
  "cpuid \n\t" /* get CPU ID flag */
  "mov %%edx, %0 \n\t" /* move result to mmx_bit */
  "popa \n\t":"=m" (flags) /* %0 */
  :"i"(0x00000001) /* %1 */

And when I write a test program, whether that assembly happens to be inside a function or not affects its output. Which seems bad. The following assembly that I found online [1] seems more robust:

#define cpuid(func,ax,bx,cx,dx)\
 __asm__ __volatile__ ("cpuid":\
 "=a" (ax), "=b" (bx), "=c" (cx), "=d" (dx) : "a" (func));
int a, b, c, d;
cpuid(1, a, b, c, d);

But I don't know enough assembly to propose a patch to the original code or explain why it is failing.

[1] http://softpixel.com/~cwright/programming/simd/cpuid.php

Steve Langasek (vorlon) wrote :

Disassembly of the _cpuFlags function on i386:

Dump of assembler code for function _cpuFlags:
=> 0xf7425030 <+0>: sub $0x10,%esp
   0xf7425033 <+3>: pusha
   0xf7425034 <+4>: mov $0x1,%eax
   0xf7425039 <+9>: cpuid
   0xf742503b <+11>: mov %edx,0xc(%esp)
   0xf742503f <+15>: popa
   0xf7425040 <+16>: mov 0xc(%esp),%eax
   0xf7425044 <+20>: add $0x10,%esp
   0xf7425047 <+23>: ret

The problem here is that popa modifies the stack pointer, so at +16 when we try to copy back off the stack into %eax, we're copying from the wrong place - pulling an arbitrary value off the stack.

I've prepared a patch for sdlgfx that appears to correct this.

Launchpad Janitor (janitor) wrote :

This bug was fixed in the package sdlgfx - 2.0.23-1ubuntu1

sdlgfx (2.0.23-1ubuntu1) quantal; urgency=low

  * debian/patches/wrong-cpuid-mmx-detection.patch: fix mmx cpuid
    detection. Closes: #663514, LP: #1029981.
 -- Steve Langasek <email address hidden> Tue, 31 Jul 2012 01:21:22 -0700

Changed in sdlgfx (Ubuntu):
status: New → Fix Released
Steve Langasek (vorlon) on 2012-07-31
Changed in libsdl-perl (Ubuntu):
status: New → Invalid
affects: libsdl-perl (Debian) → sdlgfx (Debian)
Changed in sdlgfx (Debian):
status: Unknown → Fix Released
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