[ValueTracking] Don't do an unchecked shift in ComputeNumSignBits

Summary:
Previously we used to return a bogus result, 0, for IR like `ashr %val,
-1`.

I've also added an assert checking that `ComputeNumSignBits` at least
returns 1.  That assert found an already checked in test case where we
were returning a bad result for `ashr %val, -1`.

Fixes PR32045.

Reviewers: spatel, majnemer

Reviewed By: spatel, majnemer

Subscribers: efriedma, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D30311

llvm-svn: 296273

1 files changed, 39 insertions, 0 deletions

diff --git a/llvm/test/Transforms/IndVarSimplify/pr32045.ll b/llvm/test/Transforms/IndVarSimplify/pr32045.ll
new file mode 100644
index 00000000000..31efac3f833
--- /dev/null
+++ b/llvm/test/Transforms/IndVarSimplify/pr32045.ll
@@ -0,0 +1,39 @@
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+; This is not an IndVarSimplify bug, but the original symptom
+; manifested as one.
+
+define i32 @foo(i32 %a, i32 %b, i32 %c, i32* %sink) {
+; CHECK-LABEL: @foo(
+; CHECK:       for.end:
+; CHECK-NEXT:    [[SHR:%.*]] = ashr i32 %neg3, -1
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[SHR]]
+; CHECK-NEXT:    [[SHR1:%.*]] = ashr i32 [[SUB]], [[B:%.*]]
+; CHECK-NEXT:    [[NEG:%.*]] = xor i32 [[SHR1]], -1
+; CHECK-NEXT:    store i32 [[NEG]], i32* %sink
+;
+entry:
+  %tobool2 = icmp eq i32 %a, 0
+  br i1 %tobool2, label %exit, label %preheader
+
+preheader:
+  %neg3 = phi i32 [ %c, %entry ], [ %neg, %for.end ]
+  br label %for
+
+for:
+  %p = phi i32 [ %dec, %for ], [ 1, %preheader ]
+  %cmp = icmp sgt i32 %p, -1
+  %dec = add nsw i32 %p, -1
+  br i1 %cmp, label %for, label %for.end
+
+for.end:
+  %shr = ashr i32 %neg3, %p
+  %sub = sub nsw i32 0, %shr
+  %shr1 = ashr i32 %sub, %b
+  %neg = xor i32 %shr1, -1
+  store i32 %neg, i32* %sink
+  br i1 false, label %exit, label %preheader
+
+exit:
+  ret i32 0
+}