[PATCH] powerpc: create_zero_mask() has bad inline assembly constraint

[PATCH] powerpc: create_zero_mask() has bad inline assembly constraint

[Anton Blanchard]

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.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@vger.kernel.org>
Fixes: d0cebfa650a0 ("powerpc: word-at-a-time optimization for 64-bit Little Endian")
---

diff --git a/arch/powerpc/include/asm/word-at-a-time.h b/arch/powerpc/include/asm/word-at-a-time.h
index e4396a7..4afe66a 100644
--- a/arch/powerpc/include/asm/word-at-a-time.h
+++ b/arch/powerpc/include/asm/word-at-a-time.h
@@ -82,7 +82,7 @@ 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;
 }

Re: powerpc: create_zero_mask() has bad inline assembly constraint

[Michael Ellerman]

On Fri, 2016-29-04 at 22:29:27 UTC, Unknown sender due to SPF wrote:
> 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.
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>
> Cc: <stable@vger.kernel.org>
> Fixes: d0cebfa650a0 ("powerpc: word-at-a-time optimization for 64-bit Little Endian")

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/b4c112114aab9aff5ed4568ca5

cheers