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add a random codegen deficiency.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77598 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-07-30 16:08:58 +00:00
parent 20cc4b8cd9
commit 513503961d
1 changed files with 46 additions and 0 deletions

46
lib/Target/ARM/README.txt

@ -546,3 +546,49 @@ We need to fix constant isel for ARMv6t2 to use MOVT.
Constant island pass should make use of full range SoImm values for LEApcrel.
Be careful though as the last attempt caused infinite looping on lencod.
//===---------------------------------------------------------------------===//
Predication issue. This function:
extern unsigned array[ 128 ];
int foo( int x ) {
int y;
y = array[ x & 127 ];
if ( x & 128 )
y = 123456789 & ( y >> 2 );
else
y = 123456789 & y;
return y;
}
compiles to:
_foo:
and r1, r0, #127
ldr r2, LCPI1_0
ldr r2, [r2]
ldr r1, [r2, +r1, lsl #2]
mov r2, r1, lsr #2
tst r0, #128
moveq r2, r1
ldr r0, LCPI1_1
and r0, r2, r0
bx lr
It would be better to do something like this, to fold the shift into the
conditional move:
and r1, r0, #127
ldr r2, LCPI1_0
ldr r2, [r2]
ldr r1, [r2, +r1, lsl #2]
tst r0, #128
movne r1, r1, lsr #2
ldr r0, LCPI1_1
and r0, r1, r0
bx lr
it saves an instruction and a register.
//===---------------------------------------------------------------------===//