https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113542
Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:
What |Removed |Added
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Keywords| |missed-optimization
--- Comment #2 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
The costing code is expecting
(parallel [
(set (reg:SI 124 [ _7 ])
(ne:SI (reg:SI 122 [ _2 ])
(const_int 0 [0])))
(clobber (reg:CC 100 cc))
])
To result in the assembler output
SUBS r124, R122, #1
SBC r124, R122, r124
so really should have a cost of 8 (two insns). But for some reason the thumb2
back-end is not generating that output in this case. Overall, that means that
for bic_si_test
BIC r0, r0, r1
SUBS r1, r0, #1
SBC r0, r0, r1
is neither better nor worse than
BICS r0, r0, r1
IT ne
MOVNE r0, #1
and certainly better than
BICS r0, r0, r1
ITE ne
MOVNE r2, #1
MOVEQ r2, #0
at least when it comes to code size.
So the test is somewhat flaky, but there is a further problem with the compiler
not generating the expected sequence for NE(reg, 0) in Thumb2.
