linux-kernel - [PATCH] x86/asm/bitops: Use __builtin_clz{l|ll} to evaluate constant expressions

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Message-ID: <20230828-x86_fls-v1-1-e6a31b9f79c3@google.com>
Date:   Mon, 28 Aug 2023 11:53:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Bill Wendling <morbo@...gle.com>,
        Helge Deller <deller@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chanho Min <chanho.min@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        linux-parisc@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH] x86/asm/bitops: Use __builtin_clz{l|ll} to evaluate constant expressions

Similar to commit fdb6649ab7c1 ("x86/asm/bitops: Use __builtin_ctzl() to
evaluate constant expressions") and commit 2fcff790dcb4 ("powerpc: Use
builtin functions for fls()/__fls()/fls64()").

>From a recent discussion, I noticed that x86 is lacking an optimization
that appears in arch/powerpc/include/asm/bitops.h related to constant
folding.  If you add a BUILD_BUG_ON(__builtin_constant_p(param)) to
these functions, you'll find that there were cases where the use of
inline asm pessimized the compiler's ability to perform constant folding
resulting in runtime calculation of a value that could have been
computed at compile time.

Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
Link: https://lore.kernel.org/llvm/CAKwvOdm_y6UOnxFrAiDxou2jc8CRUvyhfH9kAdc3PG0=bEvduw@mail.gmail.com/
---
 arch/x86/include/asm/bitops.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 2edf68475fec..50e5ebf9d0a0 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -293,6 +293,9 @@ static __always_inline unsigned long variable_ffz(unsigned long word)
  */
 static __always_inline unsigned long __fls(unsigned long word)
 {
+	if (__builtin_constant_p(word))
+		return BITS_PER_LONG - 1 - __builtin_clzl(word);
+
 	asm("bsr %1,%0"
 	    : "=r" (word)
 	    : "rm" (word));
@@ -360,6 +363,9 @@ static __always_inline int fls(unsigned int x)
 {
 	int r;
 
+	if (__builtin_constant_p(x))
+		return x ? 32 - __builtin_clz(x) : 0;
+
 #ifdef CONFIG_X86_64
 	/*
 	 * AMD64 says BSRL won't clobber the dest reg if x==0; Intel64 says the
@@ -401,6 +407,9 @@ static __always_inline int fls(unsigned int x)
 static __always_inline int fls64(__u64 x)
 {
 	int bitpos = -1;
+
+	if (__builtin_constant_p(x))
+		return x ? 64 - __builtin_clzll(x) : 0;
 	/*
 	 * AMD64 says BSRQ won't clobber the dest reg if x==0; Intel64 says the
 	 * dest reg is undefined if x==0, but their CPU architect says its

---
base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
change-id: 20230828-x86_fls-cf2cc49f40f3

Best regards,
-- 
Nick Desaulniers <ndesaulniers@...gle.com>