[AArch64] Remove an overly conservative check when generating store pairs.

Store instructions do not modify register values and therefore it's safe
to form a store pair even if the source register has been read in between
the two store instructions.

Previously, the read of w1 (see below) prevented the formation of a stp.

        str      w0, [x2]
        ldr     w8, [x2, #8]
        add      w0, w8, w1
        str     w1, [x2, #4]
        ret

We now generate the following code.

        stp      w0, w1, [x2]
        ldr     w8, [x2, #8]
        add      w0, w8, w1
        ret

All correctness tests with -Ofast on A57 with Spec200x and EEMBC pass.
Performance results for SPEC2K were within noise.

llvm-svn: 239432

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
index 186e71a3307..82f77a77ab5 100644
--- a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -623,7 +623,7 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
         // and first alias with the second, we can combine the second into the
         // first.
         if (!ModifiedRegs[MI->getOperand(0).getReg()] &&
-            !UsedRegs[MI->getOperand(0).getReg()] &&
+            !(MI->mayLoad() && UsedRegs[MI->getOperand(0).getReg()]) &&
             !mayAlias(MI, MemInsns, TII)) {
           MergeForward = false;
           return MBBI;
@@ -634,7 +634,8 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
         // first and the second alias with the first, we can combine the first
         // into the second.
         if (!ModifiedRegs[FirstMI->getOperand(0).getReg()] &&
-            !UsedRegs[FirstMI->getOperand(0).getReg()] &&
+            !(FirstMI->mayLoad() &&
+              UsedRegs[FirstMI->getOperand(0).getReg()]) &&
             !mayAlias(FirstMI, MemInsns, TII)) {
           MergeForward = true;
           return MBBI;