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| author | Bob Wilson <bob.wilson@apple.com> | 2011-03-03 06:39:09 +0000 |
|---|---|---|
| committer | Bob Wilson <bob.wilson@apple.com> | 2011-03-03 06:39:09 +0000 |
| commit | ab8881accd147ff3a3d699df278664e90d467bb4 (patch) | |
| tree | c7d3c95f7d4154fff05c7d50901816f682a4f7eb | |
| parent | bfdbc1155473af599081880289036def7f92e32a (diff) | |
| download | bcm5719-llvm-ab8881accd147ff3a3d699df278664e90d467bb4.tar.gz bcm5719-llvm-ab8881accd147ff3a3d699df278664e90d467bb4.zip | |
Add a readme entry for the redundant movw issue for pr9370.
llvm-svn: 126930
| -rw-r--r-- | llvm/lib/Target/ARM/README.txt | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/llvm/lib/Target/ARM/README.txt b/llvm/lib/Target/ARM/README.txt index 9fc3fb92cb2..8ba9a27e95c 100644 --- a/llvm/lib/Target/ARM/README.txt +++ b/llvm/lib/Target/ARM/README.txt @@ -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] + +//===---------------------------------------------------------------------===// |
