[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, 19 insertions, 0 deletions

diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index ba0d30d59b6..261bfc7cedf 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -239,3 +239,22 @@ TEST(ValueTracking, GuaranteedToTransferExecutionToSuccessor) {
     Index++;
   }
 }
+
+TEST(ValueTracking, ComputeNumSignBits_PR32045) {
+  StringRef Assembly = "define i32 @f(i32 %a) { "
+                       "  %val = ashr i32 %a, -1 "
+                       "  ret i32 %val "
+                       "} ";
+
+  LLVMContext Context;
+  SMDiagnostic Error;
+  auto M = parseAssemblyString(Assembly, Error, Context);
+  assert(M && "Bad assembly?");
+
+  auto *F = M->getFunction("f");
+  assert(F && "Bad assembly?");
+
+  auto *RVal =
+      cast<ReturnInst>(F->getEntryBlock().getTerminator())->getOperand(0);
+  EXPECT_EQ(ComputeNumSignBits(RVal, M->getDataLayout()), 1);
+}