Local optimization fixes for diverences found by self verification.

1) Only optimize each block once. Strictly speaking this is not a
correctness issue however it triggers the subsequent problem.

2) Ignore dead instructions.

    1: ldr     r2, [r5, #8]
    2: ldr     r3, [r5, #8](nop)
    3: str     r3, [r5, #4]
    4: movs    r3, r2

When using instruction 1 to initiate redundant ld/st eliminations, if
instruction 2 (which is already dead) is not ignored, it will be turned
into a "mov r3, r2" and that will clobber r3 used by the str.

Change-Id: I6b9a88d3688889d917b90f4b8f55278df1701c6a

1 files changed, 10 insertions, 0 deletions

diff --git a/vm/compiler/codegen/arm/LocalOptimizations.cpp b/vm/compiler/codegen/arm/LocalOptimizations.cpp
index 4c0354ac8..ffeaa576f 100644
--- a/vm/compiler/codegen/arm/LocalOptimizations.cpp
+++ b/vm/compiler/codegen/arm/LocalOptimizations.cpp
@@ -131,6 +131,12 @@ static void applyLoadStoreElimination(CompilationUnit *cUnit,
              checkLIR != tailLIR;
              checkLIR = NEXT_LIR(checkLIR)) {
 
+            /*
+             * Skip already dead instructions (whose dataflow information is
+             * outdated and misleading).
+             */
+            if (checkLIR->flags.isNop) continue;
+
             u8 checkMemMask = (checkLIR->useMask | checkLIR->defMask) &
                               ENCODE_MEM;
             u8 aliasCondition = thisMemMask & checkMemMask;
@@ -312,6 +318,10 @@ static void applyLoadHoisting(CompilationUnit *cUnit,
              checkLIR != headLIR;
              checkLIR = PREV_LIR(checkLIR)) {
 
+            /*
+             * Skip already dead instructions (whose dataflow information is
+             * outdated and misleading).
+             */
             if (checkLIR->flags.isNop) continue;
 
             u8 checkMemMask = checkLIR->defMask & ENCODE_MEM;