Re: [PATCH 46/74] x86, lto: Disable fancy hweightoptimizations for LTO

[Jan Beulich]

>>> Andi Kleen <andi@xxxxxxxxxxxxxx> 08/19/12 4:58 AM >>>
>--- a/arch/x86/Kconfig
>+++ b/arch/x86/Kconfig
>@@ -224,8 +224,9 @@ config X86_32_LAZY_GS
 >
>config ARCH_HWEIGHT_CFLAGS
>    string
>-    default "-fcall-saved-ecx -fcall-saved-edx" if X86_32
>-    default "-fcall-saved-rdi -fcall-saved-rsi -fcall-saved-rdx -fcall-saved-rcx -fcall-saved-r8 -fcall-saved-r9 -fcall-saved-r10 -fcall-saved-r11" if X86_64
>+    default "-fcall-saved-ecx -fcall-saved-edx" if X86_32 && !LTO
>+    default "-fcall-saved-rdi -fcall-saved-rsi -fcall-saved-rdx -fcall-saved-rcx -fcall-saved-r8 -fcall-saved-r9 -fcall-saved-r10 -fcall-saved-r11" if X86_64 && !LTO
>+    default "" if LTO
 
By moving this last line first you can avoid modifying the other two lines.

>--- a/arch/x86/include/asm/arch_hweight.h
>+++ b/arch/x86/include/asm/arch_hweight.h
>@@ -25,9 +25,14 @@ static inline unsigned int __arch_hweight32(unsigned int w)
>{
>    unsigned int res = 0;
 >
>+#ifdef CONFIG_LTO
>+    res  = __sw_hweight32(w);
>+#else
>+
>    asm (ALTERNATIVE("call __sw_hweight32", POPCNT32, X86_FEATURE_POPCNT)
>             : "="REG_OUT (res)
>             : REG_IN (w));
>+#endif
 
Isn't this a little to harsh? Rather than not using popcnt at all, why don't you just add
the necessary clobbers to the asm() in the LTO case?

Jan

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