linux-kernel - [PATCH 3.19.y-ckt 36/54] powerpc: Fix bad inline asm constraint in create_zero

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Message-Id: <1462838713-31343-37-git-send-email-kamal@canonical.com>
Date:	Mon,  9 May 2016 17:04:55 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Anton Blanchard <anton@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.19.y-ckt 36/54] powerpc: Fix bad inline asm constraint in create_zero_mask()

3.19.8-ckt21 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Anton Blanchard <anton@...ba.org>

commit b4c112114aab9aff5ed4568ca5e662bb02cdfe74 upstream.

In create_zero_mask() we have:

	addi	%1,%2,-1
	andc	%1,%1,%2
	popcntd	%0,%1

using the "r" constraint for %2. r0 is a valid register in the "r" set,
but addi X,r0,X turns it into an li:

	li	r7,-1
	andc	r7,r7,r0
	popcntd	r4,r7

Fix this by using the "b" constraint, for which r0 is not a valid
register.

This was found with a kernel build using gcc trunk, narrowed down to
when -frename-registers was enabled at -O2. It is just luck however
that we aren't seeing this on older toolchains.

Thanks to Segher for working with me to find this issue.

Fixes: d0cebfa650a0 ("powerpc: word-at-a-time optimization for 64-bit Little Endian")
Signed-off-by: Anton Blanchard <anton@...ba.org>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 arch/powerpc/include/asm/word-at-a-time.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/word-at-a-time.h b/arch/powerpc/include/asm/word-at-a-time.h
index 5b3a903..7043539 100644
--- a/arch/powerpc/include/asm/word-at-a-time.h
+++ b/arch/powerpc/include/asm/word-at-a-time.h
@@ -77,7 +77,7 @@ static inline unsigned long create_zero_mask(unsigned long bits)
 	    "andc	%1,%1,%2\n\t"
 	    "popcntd	%0,%1"
 		: "=r" (leading_zero_bits), "=&r" (trailing_zero_bit_mask)
-		: "r" (bits));
+		: "b" (bits));
 
 	return leading_zero_bits;
 }
-- 
2.7.4