[NFC][InstSimplify] Add tests for add nuw %x, -1 -> -1 fold.

%ret = add nuw i8 %x, C
From langref:
	nuw and nsw stand for “No Unsigned Wrap” and “No Signed Wrap”,
	respectively. If the nuw and/or nsw keywords are present,
	the result value of the add is a poison value if unsigned
	and/or signed overflow, respectively, occurs.

So if C is -1, %x can only be 0, and the result is always -1.

https://rise4fun.com/Alive/sldC
Was mentioned in D47428 review.

llvm-svn: 334236

1 files changed, 145 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll b/llvm/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll
new file mode 100644
index 00000000000..68db8cef8eb
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll
@@ -0,0 +1,145 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instsimplify -S | FileCheck %s
+
+; %ret = add nuw i8 %x, C
+; nuw means no unsigned wrap, from -1 to 0.
+; So if C is -1, %x can only be 0, and the result is always -1.
+
+define i8 @add_nuw (i8 %x) {
+; CHECK-LABEL: @add_nuw(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X:%.*]], -1
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nuw i8 %x, -1
+  ; nuw here means that %x can only be 0
+  ret i8 %ret
+}
+
+define i8 @add_nuw_nsw (i8 %x) {
+; CHECK-LABEL: @add_nuw_nsw(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw nsw i8 [[X:%.*]], -1
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nuw nsw i8 %x, -1
+  ; nuw here means that %x can only be 0
+  ret i8 %ret
+}
+
+define i8 @add_nuw_commute (i8 %x) {
+; CHECK-LABEL: @add_nuw_commute(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 -1, [[X:%.*]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nuw i8 -1, %x ; swapped
+  ; nuw here means that %x can only be 0
+  ret i8 %ret
+}
+
+; ============================================================================ ;
+; Positive tests with value range known
+; ============================================================================ ;
+
+declare void @llvm.assume(i1 %cond);
+
+define i8 @knownbits_allones(i8 %x, i8 %y) {
+; CHECK-LABEL: @knownbits_allones(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[Y:%.*]], -2
+; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP]])
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X:%.*]], [[Y]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %cmp = icmp slt i8 %y, 254
+  tail call void @llvm.assume(i1 %cmp)
+  %ret = add nuw i8 %x, %y
+  ret i8 %ret
+}
+
+; ============================================================================ ;
+; Vectors
+; ============================================================================ ;
+
+define <2 x i8> @add_vec(<2 x i8> %x) {
+; CHECK-LABEL: @add_vec(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw <2 x i8> [[X:%.*]], <i8 -1, i8 -1>
+; CHECK-NEXT:    ret <2 x i8> [[RET]]
+;
+  %ret = add nuw <2 x i8> %x, <i8 -1, i8 -1>
+  ret <2 x i8> %ret
+}
+
+define <3 x i8> @add_vec_undef(<3 x i8> %x) {
+; CHECK-LABEL: @add_vec_undef(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw <3 x i8> [[X:%.*]], <i8 -1, i8 undef, i8 -1>
+; CHECK-NEXT:    ret <3 x i8> [[RET]]
+;
+  %ret = add nuw <3 x i8> %x, <i8 -1, i8 undef, i8 -1>
+  ret <3 x i8> %ret
+}
+
+; ============================================================================ ;
+; Negative tests. Should not be folded.
+; ============================================================================ ;
+
+define i8 @bad_add (i8 %x) {
+; CHECK-LABEL: @bad_add(
+; CHECK-NEXT:    [[RET:%.*]] = add i8 [[X:%.*]], -1
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add i8 %x, -1 ; need nuw
+  ret i8 %ret
+}
+
+define i8 @bad_add_nsw (i8 %x) {
+; CHECK-LABEL: @bad_add_nsw(
+; CHECK-NEXT:    [[RET:%.*]] = add nsw i8 [[X:%.*]], -1
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nsw i8 %x, -1 ; need nuw
+  ret i8 %ret
+}
+
+; Second `add` operand is not `-1` constant
+
+define i8 @bad_add0(i8 %x, i8 %addop2) {
+; CHECK-LABEL: @bad_add0(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X:%.*]], [[ADDOP2:%.*]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nuw i8 %x, %addop2
+  ret i8 %ret
+}
+
+; Bad constant
+
+define i8 @bad_add1(i8 %x) {
+; CHECK-LABEL: @bad_add1(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X:%.*]], 1
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %ret = add nuw i8 %x, 1 ; not -1
+  ret i8 %ret
+}
+
+define <2 x i8> @bad_add_vec_nonsplat(<2 x i8> %x) {
+; CHECK-LABEL: @bad_add_vec_nonsplat(
+; CHECK-NEXT:    [[RET:%.*]] = add nuw <2 x i8> [[X:%.*]], <i8 -1, i8 1>
+; CHECK-NEXT:    ret <2 x i8> [[RET]]
+;
+  %ret = add nuw <2 x i8> %x, <i8 -1, i8 1>
+  ret <2 x i8> %ret
+}
+
+; Bad known bits
+
+define i8 @bad_knownbits(i8 %x, i8 %y) {
+; CHECK-LABEL: @bad_knownbits(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[X:%.*]], -3
+; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP]])
+; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X]], [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %cmp = icmp slt i8 %x, 253
+  tail call void @llvm.assume(i1 %cmp)
+  %ret = add nuw i8 %x, %y
+  ret i8 %ret
+}