This bug report was migrated from our old Bugzilla tracker.

These attachments are available in the static archive:

• Proposed patch for SDL_cpuinfo.c (SDL_cpuinfo.patch, text/plain, 2009-04-26 21:46:26 +0000, 1742 bytes)

Reported in version: HG 2.0
Reported for operating system, platform: Linux, x86

Comments on the original bug report:

On 2009-04-26 21:34:05 +0000, Nicholas Phillips wrote:

I am using x64 Linux (using Intel Core 2 DUO), and I have noticed that there is an error in SDL_cpuinfo.c, function CPU_getCPUIDFeaturesExt for my platform.

The code in question is:

#elif defined(GNUC) && defined (x86_64)
asm (
" movq %%rbx,%%rdi\n"
" movl $0x80000000,%%eax # Query for extended functions \n"
" cpuid # Get extended function limit \n"
" cmpl $0x80000001,%%eax \n"
" jl 1f # Nope, we dont have function 800000001h\n"
" movl $0x80000001,%%eax # Setup extended function 800000001h\n"
" cpuid # and get the information \n"
" movl %%edx,%0 \n"
"1: \n"
" movq %%rdi,%%rbx\n"
: "=m" (features)
:
: "%rax", "%rcx", "%rdx", "%rdi"
);

Even though %rbx is saved and restored manually, it still needs to appear on the clobbered register list, because it is modified by the cpuid instruction. If GCC chose to use %rbx to store %0 (features) it would cause big problems. For example consider the following code produced by GCC with the -S switch (code in question used out of context):

_ZN5CTest14MemberFunctionEv:
.LFB1029:
pushq %rbx
.LCFI1:
movl $10, %edx
movl $.LC0, %esi
movq %rdi, %rbx
call _ZNSs6assignEPKcm
#APP

225 "test.cpp" 1

      movq    %rbx,%rdi
    movl    $0x80000000,%eax   # Query for extended functions    
    cpuid                       # Get extended function limit     
    cmpl    $0x80000001,%eax                                     
    jl      1f                  # Nope, we dont have function 800000001h
    movl    $0x80000001,%eax   # Setup extended function 800000001h
    cpuid                       # and get the information         
    movl    %edx,48(%rbx)                                              

1:
movq %rdi,%rbx

0 "" 2

#NO_APP

This causes a segmentation fault for obvious reasons.

Adding %rbx to the clobbered register list makes GCC use a different register to store %0, and eliminates the problem.

Cheers.

On 2009-04-26 21:46:26 +0000, Nicholas Phillips wrote:

Created attachment 324
Proposed patch for SDL_cpuinfo.c

I noticed that the same problem could happen in other functions and for other platforms found in SDL_cpuinfo.c. Here is a simple patch.

On 2009-06-25 10:56:27 +0000, Ozkan Sezer wrote:

Is SDL-1.2 not affected?

On 2009-09-26 03:05:47 +0000, Sam Lantinga wrote:

Thank you very much for your patch!

Do you give me permission to release your code with SDL 1.3 and future
versions of SDL under both the LGPL and a closed-source commercial
license?

On 2009-09-26 09:35:19 +0000, Joshua Root wrote:

Building on i386 OS X seem to be broken now:
./src/cpuinfo/SDL_cpuinfo.c: In function 'SDL_GetCPUFeatures':
./src/cpuinfo/SDL_cpuinfo.c:150: error: PIC register '%ebx' clobbered in 'asm'

On 2009-09-26 14:40:57 +0000, Sam Lantinga wrote:

Whoops, fixed, thanks!

On 2009-10-29 21:46:02 +0000, Sam Lantinga wrote:

Okay, I tracked this down and took care of it, thanks!