powerpc: create_zero_mask() has bad inline assembly constraint
PATCH 1/1 powerpc: create_zero_mask() has bad inline assembly constraint
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.
---
arch/powerpc/include/asm/word-at-a-time.h +1 -1
@@ -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; }
Quoted 20 lines · Unknown sender due to SPF
In
create_zero_mask()we have:addi %1,%2,-1 andc %1,%1,%2 popcntd %0,%1using 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,r7Fix 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.
Quoted 5 lines
Thanks to Segher for working with me to find this issue.
Signed-off-by Anton Blanchard <[email protected]>Cc <[email protected]>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