linux-kernel - Re: [PATCH 46/74] x86, lto: Disable fancy hweight optimizations for LTO

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Date:	Sun, 19 Aug 2012 09:28:05 +0100
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<andi@...stfloor.org>, <ak@...ux.intel.com>,
	<linux-kernel@...r.kernel.org>
Cc:	<x86@...nel.org>, <akpm@...ux-foundation.org>, <mmarek@...e.cz>,
	<linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH 46/74] x86, lto: Disable fancy hweight
 optimizations for LTO

>>> Andi Kleen <andi@...stfloor.org> 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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