diff options
| 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 | acc9e7315c450ca7cffd4a65c65e578211f9d945 (patch) | |
| tree | 143610201d83da150635521b024df207f662dad2 | |
| parent | 2c11eb333c37edd5b15c1d05394861fdfc0eab90 (diff) | |
| download | llvm-acc9e7315c450ca7cffd4a65c65e578211f9d945.tar.gz llvm-acc9e7315c450ca7cffd4a65c65e578211f9d945.tar.bz2 llvm-acc9e7315c450ca7cffd4a65c65e578211f9d945.tar.xz | |
Add a readme entry for the redundant movw issue for pr9370.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126930 91177308-0d34-0410-b5e6-96231b3b80d8
| -rw-r--r-- | lib/Target/ARM/README.txt | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/lib/Target/ARM/README.txt b/lib/Target/ARM/README.txt index 9fc3fb92cb..8ba9a27e95 100644 --- a/lib/Target/ARM/README.txt +++ b/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] + +//===---------------------------------------------------------------------===// |