[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

1 files changed, 81 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/implied-cond.ll b/llvm/test/Transforms/SimplifyCFG/implied-cond.ll
new file mode 100644
index 00000000000..317adc4c347
--- /dev/null
+++ b/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)
+