[AArch64] Re-run load/store optimizer after aggressive tail duplication

The Load/Store Optimizer runs before Machine Block Placement. At O3 the
Tail Duplication Threshold is set to 4 instructions and this can create
new opportunities for the Load/Store Optimizer. It seems worthwhile to
run it once again.

llvm-svn: 349338

llvm/lib/Target/AArch64/AArch64TargetMachine.cpp

@@ -556,6 +556,12 @@ void AArch64PassConfig::addPreSched2() {
 }
 
 void AArch64PassConfig::addPreEmitPass() {
+  // Machine Block Placement might have created new opportunities when run
+  // at O3, where the Tail Duplication Threshold is set to 4 instructions.
+  // Run the load/store optimizer once more.
+  if (TM->getOptLevel() >= CodeGenOpt::Aggressive && EnableLoadStoreOpt)
+    addPass(createAArch64LoadStoreOptimizationPass());
+
   if (EnableA53Fix835769)
     addPass(createAArch64A53Fix835769());
   // Relax conditional branch instructions if they're otherwise out of

llvm/test/CodeGen/AArch64/O3-pipeline.ll

@@ -154,6 +154,7 @@
 ; CHECK-NEXT:       Machine Block Frequency Analysis
 ; CHECK-NEXT:       MachinePostDominator Tree Construction
 ; CHECK-NEXT:       Branch Probability Basic Block Placement
+; CHECK-NEXT:       AArch64 load / store optimization pass
 ; CHECK-NEXT:       Branch relaxation pass
 ; CHECK-NEXT:       AArch64 Branch Targets
 ; CHECK-NEXT:       AArch64 Compress Jump Tables

llvm/test/CodeGen/AArch64/ldst-opt-after-block-placement.ll

@@ -0,0 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O3 -mtriple=aarch64-arm < %s | FileCheck %s
+
+; Run at O3 to make sure we can optimize load/store instructions after Machine
+; Block Placement takes place using Tail Duplication Threshold = 4.
+
+define void @foo(i1 %cond, i64* %ptr) {
+; CHECK-LABEL: foo:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    tbz w0, #0, .LBB0_2
+; CHECK-NEXT:  // %bb.1: // %if.then
+; CHECK-NEXT:    ldp x9, x8, [x1, #8]
+; CHECK-NEXT:    str xzr, [x1, #16]
+; CHECK-NEXT:    cmp x8, x9
+; CHECK-NEXT:    b.lt .LBB0_3
+; CHECK-NEXT:    b .LBB0_4
+; CHECK-NEXT:  .LBB0_2: // %if.else
+; CHECK-NEXT:    ldp x8, x9, [x1]
+; CHECK-NEXT:    cmp x8, x9
+; CHECK-NEXT:    b.ge .LBB0_4
+; CHECK-NEXT:  .LBB0_3: // %exit1
+; CHECK-NEXT:    str xzr, [x1, #8]
+; CHECK-NEXT:  .LBB0_4: // %exit2
+; CHECK-NEXT:    ret
+entry:
+  br i1 %cond, label %if.then, label %if.else
+
+if.then:
+  %0 = getelementptr inbounds i64, i64* %ptr, i64 2
+  %1 = load i64, i64* %0, align 8
+  store i64 0, i64* %0, align 8
+  br label %if.end
+
+if.else:
+  %2 = load i64, i64* %ptr, align 8
+  br label %if.end
+
+if.end:
+  %3 = phi i64 [ %1, %if.then ], [ %2, %if.else ]
+  %4 = getelementptr inbounds i64, i64* %ptr, i64 1
+  %5 = load i64, i64* %4, align 8
+  %6 = icmp slt i64 %3, %5
+  br i1 %6, label %exit1, label %exit2
+
+exit1:
+  store i64 0, i64* %4, align 8
+  ret void
+
+exit2:
+  ret void
+}