[SimplifyCFG][NFC] Test that we fail to flatten CFG in JPEG "sign" value extend pattern

This comes up in JPEG decoding, see e.g.
Figure F.12 – Extending the sign bit of a decoded value in V
of ITU T.81 (JPEG specification).

llvm-svn: 366750

diff --git a/llvm/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll b/llvm/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll
new file mode 100644 (file)
index 0000000..ea013ef
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; This looks like a normal sign extension, but it's not.
+; This comes up in JPEG decoding. This check is really unpredictable,
+; so it is likely beneficial to avoid the branch.
+
+define i32 @extend_value(i32 %storage, i32 %nbits) {
+; CHECK-LABEL: @extend_value(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[SKIPNBITS:%.*]] = sub i32 32, [[NBITS:%.*]]
+; CHECK-NEXT:    [[VALUE:%.*]] = lshr i32 [[STORAGE:%.*]], [[SKIPNBITS]]
+; CHECK-NEXT:    [[SHOULDEXTEND:%.*]] = icmp sgt i32 [[STORAGE]], -1
+; CHECK-NEXT:    br i1 [[SHOULDEXTEND]], label [[EXTEND:%.*]], label [[END:%.*]]
+; CHECK:       extend:
+; CHECK-NEXT:    [[HIGHBITMASK:%.*]] = shl nsw i32 -1, [[NBITS]]
+; CHECK-NEXT:    [[HIGHBITMASKPLUSONE:%.*]] = add nsw i32 [[HIGHBITMASK]], 1
+; CHECK-NEXT:    [[EXTENDED:%.*]] = add i32 [[HIGHBITMASKPLUSONE]], [[VALUE]]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[RES:%.*]] = phi i32 [ [[EXTENDED]], [[EXTEND]] ], [ [[VALUE]], [[BB:%.*]] ]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+bb:
+  %skipnbits = sub i32 32, %nbits
+  %value = lshr i32 %storage, %skipnbits
+  %shouldextend = icmp sgt i32 %storage, -1
+  br i1 %shouldextend, label %extend, label %end
+
+extend:                                              ; preds = %bb
+  %highbitmask = shl nsw i32 -1, %nbits
+  %highbitmaskplusone = add nsw i32 %highbitmask, 1
+  %extended = add i32 %highbitmaskplusone, %value
+  br label %end
+
+end:                                              ; preds = %extend, %bb
+  %res = phi i32 [ %extended, %extend ], [ %value, %bb ]
+  ret i32 %res
+}