[SystemZ] Improve optimizeCompareZero()

More conversions to load-and-test can be made with this patch by adding a
forward search in optimizeCompareZero().

Review: Ulrich Weigand
https://reviews.llvm.org/D38076

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313877 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/SystemZ/SystemZElimCompare.cpp

@@ -110,12 +110,8 @@ static bool isCCLiveOut(MachineBasicBlock &MBB) {
   return false;
 }
 
-// Return true if any CC result of MI would reflect the value of Reg.
-static bool resultTests(MachineInstr &MI, unsigned Reg) {
-  if (MI.getNumOperands() > 0 && MI.getOperand(0).isReg() &&
-      MI.getOperand(0).isDef() && MI.getOperand(0).getReg() == Reg)
-    return true;
-
+// Returns true if MI is an instruction whose output equals the value in Reg.
+static bool preservesValueOf(MachineInstr &MI, unsigned Reg) {
   switch (MI.getOpcode()) {
   case SystemZ::LR:
   case SystemZ::LGR:
@@ -136,6 +132,16 @@ static bool resultTests(MachineInstr &MI, unsigned Reg) {
   return false;
 }
 
+// Return true if any CC result of MI would (perhaps after conversion)
+// reflect the value of Reg.
+static bool resultTests(MachineInstr &MI, unsigned Reg) {
+  if (MI.getNumOperands() > 0 && MI.getOperand(0).isReg() &&
+      MI.getOperand(0).isDef() && MI.getOperand(0).getReg() == Reg)
+    return true;
+
+  return (preservesValueOf(MI, Reg));
+}
+
 // Describe the references to Reg or any of its aliases in MI.
 Reference SystemZElimCompare::getRegReferences(MachineInstr &MI, unsigned Reg) {
   Reference Ref;
@@ -421,11 +427,34 @@ bool SystemZElimCompare::optimizeCompareZero(
     }
     SrcRefs |= getRegReferences(MI, SrcReg);
     if (SrcRefs.Def)
-      return false;
+      break;
     CCRefs |= getRegReferences(MI, SystemZ::CC);
     if (CCRefs.Use && CCRefs.Def)
+      break;
+  }
+
+  // Also do a forward search to handle cases where an instruction after the
+  // compare can be converted like
+  //
+  // LTEBRCompare %F0S, %F0S, %CC<imp-def> LTEBRCompare %F0S, %F0S, %CC<imp-def>
+  // %F2S<def> = LER %F0S
+  //
+  MBBI = Compare, MBBE = MBB.end();
+  while (++MBBI != MBBE) {
+    MachineInstr &MI = *MBBI;
+    if (preservesValueOf(MI, SrcReg)) {
+      // Try to eliminate Compare by reusing a CC result from MI.
+      if (convertToLoadAndTest(MI)) {
+        EliminatedComparisons += 1;
+        return true;
+      }
+    }
+    if (getRegReferences(MI, SrcReg).Def)
+      return false;
+    if (getRegReferences(MI, SystemZ::CC))
       return false;
   }
+
   return false;
 }
 

test/CodeGen/SystemZ/fp-cmp-07.mir

@@ -0,0 +1,44 @@
+# RUN: llc -mtriple=s390x-linux-gnu -mcpu=z10 -no-integrated-as -start-after=block-placement %s -o - | FileCheck %s
+# Test that LTEBR is used without an unnecessary LER
+
+--- |
+  define float @f15(float %val, float %dummy, float* %dest) {
+  entry:
+    call void asm sideeffect "blah $0", "{f2}"(float %val)
+    %cmp = fcmp olt float %val, 0.000000e+00
+    br i1 %cmp, label %exit, label %store
+
+  store:                                            ; preds = %entry
+    store float %val, float* %dest
+    br label %exit
+
+  exit:                                             ; preds = %store, %entry
+    ret float %val
+  }
+
+...
+
+# CHECK: ltebr %f2, %f0
+
+---
+name:            f15
+tracksRegLiveness: true
+liveins:
+  - { reg: '%f0s', virtual-reg: '' }
+  - { reg: '%r2d', virtual-reg: '' }
+body:             |
+  bb.0.entry:
+    liveins: %f0s, %r2d
+
+    LTEBRCompare %f0s, %f0s, implicit-def %cc
+    %f2s = LER %f0s
+    INLINEASM $"blah $0", 1, 9, %f2s
+    CondReturn 15, 4, implicit %f0s, implicit %cc
+
+  bb.1.store:
+    liveins: %f0s, %r2d
+
+    STE %f0s, killed %r2d, 0, _ :: (store 4 into %ir.dest)
+    Return implicit %f0s
+
+...