[PATCH] [JumpThreading] Simplify comparisons when simplifying branches

[Philip Reames]

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9312

Files:
  llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/trunk/test/Transforms/JumpThreading/thread-cmp.ll

Index: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
@@ -791,6 +791,17 @@
           CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
           BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
           CondBr->eraseFromParent();
+          if (CondCmp->use_empty())
+            CondCmp->eraseFromParent();
+          else if (CondCmp->getParent() == BB) {
+            // If the fact we just learned is true for all uses of the
+            // condition, replace it with a constant value
+            auto *CI = Baseline == LazyValueInfo::True ?
+              ConstantInt::getTrue(CondCmp->getType()) :
+              ConstantInt::getFalse(CondCmp->getType());
+            CondCmp->replaceAllUsesWith(CI);
+            CondCmp->eraseFromParent();
+          }
           return true;
         }
       }
Index: llvm/trunk/test/Transforms/JumpThreading/thread-cmp.ll
===================================================================
--- llvm/trunk/test/Transforms/JumpThreading/thread-cmp.ll
+++ llvm/trunk/test/Transforms/JumpThreading/thread-cmp.ll
@@ -0,0 +1,69 @@
+; RUN: opt -S -jump-threading %s | FileCheck %s
+; When simplify a branch based on LVI predicates, we should replace the 
+; comparison itself with a constant (when possible) in case it's otherwise used.
+
+define i32 @test(i32* %p) {
+; CHECK-LABEL: @test
+; CHECK: icmp eq
+; CHECK-NEXT: br i1 %cmp, label %exit2, label %exit1
+; CHECK-NOT: icmp ne
+entry:
+  %cmp = icmp eq i32* %p, null
+  br i1 %cmp, label %is_null, label %not_null
+is_null:
+  %cmp2 = icmp ne i32* %p, null
+  br i1 %cmp2, label %exit1, label %exit2
+not_null:
+  %cmp3 = icmp ne i32* %p, null
+  br i1 %cmp3, label %exit1, label %exit2
+exit1:
+  ret i32 0
+exit2:
+  ret i32 1
+}
+
+declare void @use(i1)
+
+; It would not be legal to replace %cmp2 (well, in this case it actually is, 
+; but that's a CSE problem, not a LVI/jump threading problem)
+define i32 @test_negative(i32* %p) {
+; CHECK-LABEL: @test
+; CHECK: icmp ne
+; CHECK: icmp eq
+; CHECK-NEXT: br i1 %cmp, label %exit2, label %exit1
+; CHECK-NOT: icmp ne
+entry:
+  %cmp2 = icmp ne i32* %p, null
+  call void @use(i1 %cmp2)
+  %cmp = icmp eq i32* %p, null
+  br i1 %cmp, label %is_null, label %not_null
+is_null:
+  br i1 %cmp2, label %exit1, label %exit2
+not_null:
+  br i1 %cmp2, label %exit1, label %exit2
+exit1:
+  ret i32 0
+exit2:
+  ret i32 1
+}
+
+; In this case, we can remove cmp2 because it's otherwise unused
+define i32 @test2(i32* %p) {
+; CHECK-LABEL: @test
+; CHECK-LABEL: entry:
+; CHECK-NEXT: icmp eq
+; CHECK-NEXT: br i1 %cmp, label %exit2, label %exit1
+; CHECK-NOT: icmp ne
+entry:
+  %cmp2 = icmp ne i32* %p, null
+  %cmp = icmp eq i32* %p, null
+  br i1 %cmp, label %is_null, label %not_null
+is_null:
+  br i1 %cmp2, label %exit1, label %exit2
+not_null:
+  br i1 %cmp2, label %exit1, label %exit2
+exit1:
+  ret i32 0
+exit2:
+  ret i32 1
+}

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