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Troubles compiling code with -fPIC [message #40190]

Sun, 30 June 2013 21:38

dolik.rce
Messages: 1789
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

Hi everyone,

I've hit a problem when trying to compile a shared library using U++ with -fPIC switch:

/home/h/upp-production/uppsrc/Core/Cpu.cpp: In function 'void Upp::sCheckCPU()':
/home/h/upp-production/uppsrc/Core/Cpu.cpp:40:84: error: PIC register clobbered by '%ebx' in 'asm'
  __asm__("movl $1, %%eax\n\tcpuid" : "=d" (info1), "=c" (info2) : : "%eax", "%ebx");

As the error message says, the problem is caused by changing a value of ebx register. After a little googling, I found that the widely used fix in this situation is to store the value of ebx before calling cpuid instruction and restore it afterwards. When I looked into the code of SCheckCpu() to do that, to my great surprise I found the code was already there, just commented out Smile

So my question is, does anyone remember if there was some valid reason to comment this line out? If not, I propose to start using it:

Index: /home/h/upp-production/uppsrc/Core/Cpu.cpp
===================================================================
--- /home/h/upp-production/uppsrc/Core/Cpu.cpp  (revision 6151)
+++ /home/h/upp-production/uppsrc/Core/Cpu.cpp  (working copy)
@@ -15,7 +15,6 @@
        if(done) return;
        done = true;
 #ifdef PLATFORM_OSX11
-//     __asm__("pushl %%ebx\n\tmovl $1, %%eax\n\tcpuid\n\tpopl %%ebx" : "=d" (info1), "=c" (info2) : : "%eax");
        sHasMMX = true;
        sHasSSE = true;
        sHasSSE2 = true;
@@ -37,7 +36,7 @@
 #else
        dword info1;
        dword info2;
-       __asm__("movl $1, %%eax\n\tcpuid" : "=d" (info1), "=c" (info2) : : "%eax", "%ebx");
+       __asm__("pushl %%ebx\n\tmovl $1, %%eax\n\tcpuid\n\tpopl %%ebx" : "=d" (info1), "=c" (info2) : : "%eax");
 #endif
        sHasMMX = ((info1 >> 23) & 0x1);
        sHasSSE = ((info1 >> 25) & 0x1);

With this patch my code compiles and I noticed no difference for other programs.

Best regards,
Honza

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Re: Troubles compiling code with -fPIC [message #40191 is a reply to message #40190]

Mon, 01 July 2013 08:46

mirek
Messages: 13975
Registered: November 2005

Ultimate Member

This really weird. Not when it was commented out, but my guess is it is commented out since the beginning:

The part of code is for MacOS X, X11 subsystem. Macs always have CPUs that support MMX/SSE/SSE2, so there is no point to call assembler to find out... so all assembly code is #ifdefed out... I doubt that adding some back is the cause of -fPIC problems...

Mirek

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