[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239432 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Chad Rosier

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;

test/CodeGen/AArch64/arm64-stp.ll

@@ -99,3 +99,35 @@ entry:
   store <4 x i32> %p20, <4 x i32>* %p21, align 4
   ret void
 }
+
+; Read of %b to compute %tmp2 shouldn't prevent formation of stp
+; CHECK-LABEL: stp_int_rar_hazard
+; CHECK: stp w0, w1, [x2]
+; CHECK: ldr [[REG:w[0-9]+]], [x2, #8]
+; CHECK: add w0, [[REG]], w1
+; CHECK: ret
+define i32 @stp_int_rar_hazard(i32 %a, i32 %b, i32* nocapture %p) nounwind {
+  store i32 %a, i32* %p, align 4
+  %ld.ptr = getelementptr inbounds i32, i32* %p, i64 2
+  %tmp = load i32, i32* %ld.ptr, align 4
+  %tmp2 = add i32 %tmp, %b
+  %add.ptr = getelementptr inbounds i32, i32* %p, i64 1
+  store i32 %b, i32* %add.ptr, align 4
+  ret i32 %tmp2
+}
+
+; Read of %b to compute %tmp2 shouldn't prevent formation of stp
+; CHECK-LABEL: stp_int_rar_hazard_after
+; CHECK: ldr [[REG:w[0-9]+]], [x3, #4]
+; CHECK: add w0, [[REG]], w2
+; CHECK: stp w1, w2, [x3]
+; CHECK: ret
+define i32 @stp_int_rar_hazard_after(i32 %w0, i32 %a, i32 %b, i32* nocapture %p) nounwind {
+  store i32 %a, i32* %p, align 4
+  %ld.ptr = getelementptr inbounds i32, i32* %p, i64 1
+  %tmp = load i32, i32* %ld.ptr, align 4
+  %tmp2 = add i32 %tmp, %b
+  %add.ptr = getelementptr inbounds i32, i32* %p, i64 1
+  store i32 %b, i32* %add.ptr, align 4
+  ret i32 %tmp2
+}