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Add a readme entry for the redundant movw issue for pr9370.

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llvm-svn: 126930
This commit is contained in:
Bob Wilson 2011-03-03 06:39:09 +00:00
parent 3511ad1023
commit 72ccdfe148
1 changed files with 24 additions and 0 deletions
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24
lib/Target/ARM/README.txt
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@ -657,3 +657,27 @@ Note that both "tst" and "moveq" are redundant.
//===---------------------------------------------------------------------===//
When loading immediate constants with movt/movw, if there are multiple
constants needed with the same low 16 bits, and those values are not live at
the same time, it would be possible to use a single movw instruction, followed
by multiple movt instructions to rewrite the high bits to different values.
For example:
volatile store i32 -1, i32* inttoptr (i32 1342210076 to i32*), align 4,
!tbaa
!0
volatile store i32 -1, i32* inttoptr (i32 1342341148 to i32*), align 4,
!tbaa
!0
is compiled and optimized to:
movw r0, #32796
mov.w r1, #-1
movt r0, #20480
str r1, [r0]
movw r0, #32796 @ <= this MOVW is not needed, value is there already
movt r0, #20482
str r1, [r0]
//===---------------------------------------------------------------------===//