== Summary ==
- benchmarking coremark with VRP based extension elimination
* extension elimination in some cases affecting other optimizations
* With this improvements are marginal (details below)
== Plan ==
- study crc where extension elimination is resulting in bad code
- Find a solution
==Details==
If an assignment gimple statement has RHS expression value that can fit
in LHS type, truncation is redundant. Zero/sign extensions are redundant
in this case and rtl statement can be replaced as
from:
(insn 12 11 0 (set (reg:SI 110 [ D.4128 ])
(zero_extend:SI (subreg:HI (reg:SI 117) 0))) c5.c:8 -1
(nil))
to:
(insn 12 11 0 (set (subreg/s/u:HI (reg:SI 110 [ D.4128 ]) 0)
(subreg:HI (reg:SI 117) 0)) c5.c:8 -1
(nil))
With this change, for the following case:
short unPack( unsigned char c )
{
/* Only want lower four bit nibble */
c = c & (unsigned char)0x0F ;
if( c > 7 ) {
/* Negative nibble */
return( ( short )( c - 16 ) ) ;
}
else
{
/* positive nibble */
return( ( short )c ) ;
}
}
asm without elimination
unPack:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
and r0, r0, #15
cmp r0, #7
subhi r0, r0, #16
uxthhi r0, r0
sxth r0, r0
bx lr
.size
asm with elimination
unPack:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
and r0, r0, #15
cmp r0, #7
subhi r0, r0, #16
sxth r0, r0
bx lr
In some cases, changed rtl statement is not eliminated by later passes
and is generated as a mov instruction. Worse, it also seems to affect
the other optimization passes and resulting in worse code for crc. Not
found the cause for it yet.
_______________________________________________
linaro-toolchain mailing list
linaro-toolchain@lists.linaro.org
http://lists.linaro.org/mailman/listinfo/linaro-toolchain
