[vlc-commits] [Git][videolan/vlc][master] 5 commits: cpu: remove gratuitious swap

[Rémi Denis-Courmont (@Courmisch)]

Rémi Denis-Courmont pushed to branch master at VideoLAN / VLC


Commits:
c0901dbc by Rémi Denis-Courmont at 2022-02-28T17:49:28+00:00
cpu: remove gratuitious swap

This gets the EBX value directly from, well, EBX. The compiler knows
that EBX will be clobbered given it is an output operand, so it will
save and restore it if/as needed.

- - - - -
4ea8f779 by Rémi Denis-Courmont at 2022-02-28T17:49:28+00:00
cpu: remove volatile qualifier

We do not care about any side effects here, only the output operands.

- - - - -
a0356a20 by Rémi Denis-Courmont at 2022-02-28T17:49:28+00:00
cpu: remove volatile qualifier

- - - - -
675aeb02 by Rémi Denis-Courmont at 2022-02-28T17:49:28+00:00
cpu: output old flags straight

Output the current CPU flags directly into the selected register.
Thus EBX is left alone, and need not be saved/restored.

- - - - -
c12fc2ad by Rémi Denis-Courmont at 2022-02-28T17:49:28+00:00
cpu: do not hard-code EAX

Let the compiler pick whichever register it sees fit.

- - - - -


1 changed file:

- src/misc/cpu.c


Changes:

=====================================
src/misc/cpu.c
=====================================
@@ -118,43 +118,32 @@ VLC_WEAK unsigned vlc_CPU_raw(void)
     unsigned int i_eax, i_ebx, i_ecx, i_edx;
 
     /* Needed for x86 CPU capabilities detection */
-# if defined (__i386__) && defined (__PIC__)
-#  define cpuid(reg) \
-    asm volatile ("xchgl %%ebx,%1\n\t" \
-                  "cpuid\n\t" \
-                  "xchgl %%ebx,%1\n\t" \
-                  : "=a" (i_eax), "=r" (i_ebx), "=c" (i_ecx), "=d" (i_edx) \
-                  : "a" (reg) \
-                  : "cc");
-# else
-#  define cpuid(reg) \
-    asm volatile ("cpuid\n\t" \
-                  : "=a" (i_eax), "=b" (i_ebx), "=c" (i_ecx), "=d" (i_edx) \
-                  : "a" (reg) \
-                  : "cc");
-# endif
+# define cpuid(reg) \
+    asm ("cpuid" \
+         : "=a" (i_eax), "=b" (i_ebx), "=c" (i_ecx), "=d" (i_edx) \
+         : "a" (reg) \
+         : "cc");
+
      /* Check if the OS really supports the requested instructions */
 # if defined (__i386__) && !defined (__i586__) \
   && !defined (__i686__) && !defined (__pentium4__) \
   && !defined (__k6__) && !defined (__athlon__) && !defined (__k8__)
 #  if !defined (__i486__)
-    /* check if cpuid instruction is supported */
-   asm volatile ("push %%ebx\n\t"
-                 "pushf\n\t"
-                 "pop %%eax\n\t"
-                 "movl %%eax, %%ebx\n\t"
-                 "xorl $0x200000, %%eax\n\t"
-                 "push %%eax\n\t"
-                 "popf\n\t"
-                 "pushf\n\t"
-                 "pop %%eax\n\t"
-                 "movl %%ebx,%1\n\t"
-                 "pop %%ebx\n\t"
-                 : "=a" (i_eax), "=r" (i_ebx)
-                 :
-                 : "cc" );
-
-    if( i_eax == i_ebx )
+    /* Check if CPUID is supported by setting ID flag bit 21. */
+    unsigned int after, before;
+    asm ("pushf\n\t"
+         "pop %0\n\t"
+         "movl %0, %1\n\t"
+         "xorl $0x200000, %0\n\t"
+         "push %0\n\t"
+         "popf\n\t"
+         "pushf\n\t"
+         "pop %0\n\t"
+         : "=&r" (after), "=&r" (before)
+         :
+         : "cc");
+
+    if( after == before )
         goto out;
 # endif
 



View it on GitLab: https://code.videolan.org/videolan/vlc/-/compare/fe1083cdf555bf768e2228fd2a76dc6cb0b0c434...c12fc2ad616262db6c8c39c062d60acf93247f76

-- 
View it on GitLab: https://code.videolan.org/videolan/vlc/-/compare/fe1083cdf555bf768e2228fd2a76dc6cb0b0c434...c12fc2ad616262db6c8c39c062d60acf93247f76
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