[PATCH] If a conditional branch jumps to the same target, remove the condition

[Philip Reames]

Given much of inst combine is restricted to instructions which have one use, getting rid of a use on the condition can help the effectiveness of the optimizer.  Also, it allows the condition to potentially be deleted by instcombine rather than waiting for another pass.  

I noticed this completely by accident in another test case.  It's not anything that actually came from a real workload.  

p.s. We should probably do the same thing for switch instructions.

http://reviews.llvm.org/D8220

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/branch.ll

Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2012,6 +2012,15 @@
     return &BI;
   }
 
+  // If the condition is irrelevant, remove the use so that other
+  // transforms on the condition become more effective.
+  if (BI.isConditional() &&
+      BI.getSuccessor(0) == BI.getSuccessor(1) &&
+      !isa<UndefValue>(BI.getCondition())) {
+    BI.setCondition(UndefValue::get(BI.getCondition()->getType()));
+    return &BI;
+  }
+
   // Canonicalize fcmp_one -> fcmp_oeq
   FCmpInst::Predicate FPred; Value *Y;
   if (match(&BI, m_Br(m_FCmp(FPred, m_Value(X), m_Value(Y)),
Index: test/Transforms/InstCombine/branch.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/branch.ll
@@ -0,0 +1,16 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+define i32 @test(i32 %x) {
+; CHECK-LABEL: @test
+entry:
+; CHECK-NOT: icmp
+; CHECK: br i1 undef, 
+  %cmp = icmp ult i32 %x, 7
+  br i1 %cmp, label %merge, label %merge
+merge:
+; CHECK-LABEL: merge:
+; CHECK: ret i32 %x
+  ret i32 %x
+}
+
+

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