From acc9e7315c450ca7cffd4a65c65e578211f9d945 Mon Sep 17 00:00:00 2001 From: Bob Wilson Date: Thu, 3 Mar 2011 06:39:09 +0000 Subject: [PATCH] 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 --- lib/Target/ARM/README.txt | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/lib/Target/ARM/README.txt b/lib/Target/ARM/README.txt index 9fc3fb92cb2..8ba9a27e95c 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] + +//===---------------------------------------------------------------------===//