[SimplifyCFG] Constant fold a branch implied by it's incoming edge

The most common use case is when eliminating redundant range checks in an example like the following:
c = a[i+1] + a[i];

Note that all the smarts of the transform (the implication engine) is already in ValueTracking and is tested directly through InstructionSimplify.

Differential Revision: http://reviews.llvm.org/D13040

llvm-svn: 251596

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2390,6 +2390,19 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
     if (CE->canTrap())
       return false;
 
+  // If BI is reached from the true path of PBI and PBI's condition implies
+  // BI's condition, we know the direction of the BI branch.
+  if (PBI->getSuccessor(0) == BI->getParent() &&
+      isImpliedCondition(PBI->getCondition(), BI->getCondition()) &&
+      PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
+      BB->getSinglePredecessor()) {
+    // Turn this into a branch on constant.
+    auto *OldCond = BI->getCondition();
+    BI->setCondition(ConstantInt::getTrue(BB->getContext()));
+    RecursivelyDeleteTriviallyDeadInstructions(OldCond);
+    return true;  // Nuke the branch on constant.
+  }
+
   // If this is a conditional branch in an empty block, and if any
   // predecessors are a conditional branch to one of our destinations,
   // fold the conditions into logical ops and one cond br.

llvm/test/Transforms/SimplifyCFG/implied-cond.ll

@@ -0,0 +1,81 @@
+; RUN: opt %s -S -simplifycfg | FileCheck %s
+; Check for when one branch implies the value of a successors conditional and
+; it's not simply the same conditional repeated.
+
+define void @test(i32 %length.i, i32 %i) {
+; CHECK-LABEL: @test
+  %iplus1 = add nsw i32 %i, 1
+  %var29 = icmp slt i32 %iplus1, %length.i
+; CHECK: br i1 %var29, label %in_bounds, label %out_of_bounds
+  br i1 %var29, label %next, label %out_of_bounds
+
+next:
+; CHECK-LABEL: in_bounds:
+; CHECK-NEXT: ret void
+  %var30 = icmp slt i32 %i, %length.i
+  br i1 %var30, label %in_bounds, label %out_of_bounds2
+
+in_bounds:
+  ret void
+
+out_of_bounds:
+  call void @foo(i64 0)
+  unreachable
+
+out_of_bounds2:
+  call void @foo(i64 1)
+  unreachable
+}
+
+; If the add is not nsw, it's not safe to use the fact about i+1 to imply the
+; i condition since it could have overflowed.
+define void @test_neg(i32 %length.i, i32 %i) {
+; CHECK-LABEL: @test_neg
+  %iplus1 = add i32 %i, 1
+  %var29 = icmp slt i32 %iplus1, %length.i
+; CHECK: br i1 %var29, label %next, label %out_of_bounds
+  br i1 %var29, label %next, label %out_of_bounds
+
+next:
+  %var30 = icmp slt i32 %i, %length.i
+; CHECK: br i1 %var30, label %in_bounds, label %out_of_bounds2
+  br i1 %var30, label %in_bounds, label %out_of_bounds2
+
+in_bounds:
+  ret void
+
+out_of_bounds:
+  call void @foo(i64 0)
+  unreachable
+
+out_of_bounds2:
+  call void @foo(i64 1)
+  unreachable
+}
+
+
+define void @test2(i32 %length.i, i32 %i) {
+; CHECK-LABEL: @test2
+  %iplus100 = add nsw i32 %i, 100
+  %var29 = icmp slt i32 %iplus100, %length.i
+; CHECK: br i1 %var29, label %in_bounds, label %out_of_bounds
+  br i1 %var29, label %next, label %out_of_bounds
+
+next:
+  %var30 = icmp slt i32 %i, %length.i
+  br i1 %var30, label %in_bounds, label %out_of_bounds2
+
+in_bounds:
+  ret void
+
+out_of_bounds:
+  call void @foo(i64 0)
+  unreachable
+
+out_of_bounds2:
+  call void @foo(i64 1)
+  unreachable
+}
+
+declare void @foo(i64)
+