Don't crash in SE dealing with ashr x, -1

ScalarEvolution::getSignedRange uses ComputeNumSignBits from ValueTracking on
ashr instructions. ComputeNumSignBits can return zero, but this case was not
handled correctly by the code in getSignedRange which was calling:
  APInt::getSignedMinValue(BitWidth).ashr(NS - 1)
with NS = 0, resulting in an assertion failure in APInt::ashr.

Now, we just return the conservative result (as with NS == 1).

Another bug found by llvm-stress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185955 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/test/CodeGen/PowerPC/ashr-neg1.ll b/test/CodeGen/PowerPC/ashr-neg1.ll
new file mode 100644
index 00000000000..28e74f4d298
--- /dev/null
+++ b/test/CodeGen/PowerPC/ashr-neg1.ll
@@ -0,0 +1,18 @@
+; RUN: llc -mtriple=powerpc64-unknown-linux-gnu -mcpu=pwr7 < %s
+target triple = "powerpc64-unknown-linux-gnu"
+
+define void @autogen_SD30723(i32) {
+BB:
+  br label %CF80
+
+CF80:                                             ; preds = %CF80, %BB
+  %B = ashr i32 %0, -1
+  br i1 undef, label %CF80, label %CF84
+
+CF84:                                             ; preds = %CF84, %CF80
+  %Cmp62 = icmp sge i32 undef, %B
+  br i1 %Cmp62, label %CF84, label %CF85
+
+CF85:                                             ; preds = %CF85, %CF84
+  br label %CF85
+}