[PATCH] D45378: [InstCombine] Propagate null values from conditions to other basic blocks

[Dávid Bolvanský via Phabricator via llvm-commits]

xbolva00 updated this revision to Diff 141428.

https://reviews.llvm.org/D45378

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/cmp-propagate-zero-val.ll


Index: test/Transforms/InstCombine/cmp-propagate-zero-val.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/cmp-propagate-zero-val.ll
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+ at .str = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
+
+define i8* @propagate_null(i8* %ptr) #0 {
+; CHECK-LABEL: @propagate_null(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8* [[PTR:%.*]], null
+; CHECK-NEXT:    br i1 [[TOBOOL]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[RETURN:%.*]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[PUTCHAR:%.*]] = call i32 @putchar(i32 10)
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i8* [[PTR]]
+;
+entry:
+  %retval = alloca i8*, align 4
+  %tobool = icmp ne i8* %ptr, null
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  store i8* null, i8** %retval, align 4
+  br label %return
+
+if.end:                                           ; preds = %entry
+  %call = call i32 @puts(i8* getelementptr inbounds ([1 x i8], [1 x i8]* @.str, i32 0, i32 0))
+  store i8* %ptr, i8** %retval, align 4
+  br label %return
+
+return:                                           ; preds = %if.end, %if.then
+  %0 = load i8*, i8** %retval, align 4
+  ret i8* %0
+}
+
+declare i32 @puts(i8*) #1
+
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2392,7 +2392,37 @@
     return &BI;
   }
 
-  return nullptr;
+  // Propagate null op from cmp to next basic blocks
+  if (match(&BI, m_Br(m_OneUse(m_Cmp(Pred, m_Value(), m_Zero())), TrueDest,
+                      FalseDest)) &&
+      isCanonicalPredicate(Pred)) {
+    auto *Cmp = dyn_cast<CmpInst>(BI.getOperand(0));
+    Value *NullOp = Cmp->getOperand(0); // left side of cmp eq
+    bool NullOpPropagated = false;
+    BasicBlock *Next;
+    for (BasicBlock *B = TrueDest; B; B = Next) {
+      Next = nullptr;
+      for (Instruction &I : *B) {
+        for (unsigned i = 0; i < I.getNumOperands(); ++i) {
+          Value *Arg = I.getOperand(i);
+          if (match(Arg, m_Zero()) && NullOp->getType() == Arg->getType()) {
+            I.setOperand(i, NullOp);
+            NullOpPropagated |= true;
+          }
+        }
+
+        if (BranchInst *BI = dyn_cast<BranchInst>(&I)) {
+          if (BI->isUnconditional()) {
+            Next = dyn_cast<BasicBlock>(BI->getOperand(0));
+          }
+        }
+      }
+    }
+
+    if (NullOpPropagated)
+      return &BI;
+
+    return nullptr;
 }
 
 Instruction *InstCombiner::visitSwitchInst(SwitchInst &SI) {


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