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Summary: `%ret = add nuw i8 %x, C` From [[ https://llvm.org/docs/LangRef.html#add-instruction | 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`. I'm not sure we want to use `KnownBits`/`LVI` here, because there is exactly one possible value (all bits set, `-1`), so some other pass should take care of replacing the known-all-ones with constant `-1`. The `test/Transforms/InstCombine/set-lowbits-mask-canonicalize.ll` change *is* confusing. What happening is, before this: (omitting `nuw` for simplicity) 1. First, InstCombine D47428/rL334127 folds `shl i32 1, %NBits`) to `shl nuw i32 -1, %NBits` 2. Then, InstSimplify D47883/rL334222 folds `shl nuw i32 -1, %NBits` to `-1`, 3. `-1` is inverted to `0`. But now: 1. *This* InstSimplify fold `%ret = add nuw i32 %setbit, -1` -> `-1` happens first, before InstCombine D47428/rL334127 fold could happen. Thus we now end up with the opposite constant, and it is all good: https://rise4fun.com/Alive/OA9 https://rise4fun.com/Alive/sldC Was mentioned in D47428 review. Follow-up for D47883. Reviewers: spatel, craig.topper Reviewed By: spatel Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D47908 llvm-svn: 334298
141 lines
3.7 KiB
LLVM
141 lines
3.7 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt < %s -instsimplify -S | FileCheck %s
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; %ret = add nuw i8 %x, C
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; nuw means no unsigned wrap, from -1 to 0.
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; So if C is -1, %x can only be 0, and the result is always -1.
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define i8 @add_nuw (i8 %x) {
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; CHECK-LABEL: @add_nuw(
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; CHECK-NEXT: ret i8 -1
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;
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%ret = add nuw i8 %x, -1
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; nuw here means that %x can only be 0
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ret i8 %ret
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}
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define i8 @add_nuw_nsw (i8 %x) {
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; CHECK-LABEL: @add_nuw_nsw(
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; CHECK-NEXT: ret i8 -1
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;
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%ret = add nuw nsw i8 %x, -1
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; nuw here means that %x can only be 0
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ret i8 %ret
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}
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define i8 @add_nuw_commute (i8 %x) {
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; CHECK-LABEL: @add_nuw_commute(
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; CHECK-NEXT: ret i8 -1
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;
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%ret = add nuw i8 -1, %x ; swapped
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; nuw here means that %x can only be 0
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ret i8 %ret
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}
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; ============================================================================ ;
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; Positive tests with value range known
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; ============================================================================ ;
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declare void @llvm.assume(i1 %cond);
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define i8 @knownbits_allones(i8 %x, i8 %y) {
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; CHECK-LABEL: @knownbits_allones(
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; CHECK-NEXT: [[CMP:%.*]] = icmp slt i8 [[Y:%.*]], -2
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; CHECK-NEXT: tail call void @llvm.assume(i1 [[CMP]])
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; CHECK-NEXT: [[RET:%.*]] = add nuw i8 [[X:%.*]], [[Y]]
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%cmp = icmp slt i8 %y, 254
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tail call void @llvm.assume(i1 %cmp)
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%ret = add nuw i8 %x, %y
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ret i8 %ret
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}
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; ============================================================================ ;
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; Vectors
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; ============================================================================ ;
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define <2 x i8> @add_vec(<2 x i8> %x) {
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; CHECK-LABEL: @add_vec(
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; CHECK-NEXT: ret <2 x i8> <i8 -1, i8 -1>
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;
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%ret = add nuw <2 x i8> %x, <i8 -1, i8 -1>
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ret <2 x i8> %ret
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}
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define <3 x i8> @add_vec_undef(<3 x i8> %x) {
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; CHECK-LABEL: @add_vec_undef(
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; CHECK-NEXT: ret <3 x i8> <i8 -1, i8 undef, i8 -1>
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;
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%ret = add nuw <3 x i8> %x, <i8 -1, i8 undef, i8 -1>
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ret <3 x i8> %ret
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}
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; ============================================================================ ;
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; Negative tests. Should not be folded.
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; ============================================================================ ;
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define i8 @bad_add (i8 %x) {
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; CHECK-LABEL: @bad_add(
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; CHECK-NEXT: [[RET:%.*]] = add i8 [[X:%.*]], -1
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%ret = add i8 %x, -1 ; need nuw
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ret i8 %ret
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}
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define i8 @bad_add_nsw (i8 %x) {
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; CHECK-LABEL: @bad_add_nsw(
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; CHECK-NEXT: [[RET:%.*]] = add nsw i8 [[X:%.*]], -1
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%ret = add nsw i8 %x, -1 ; need nuw
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ret i8 %ret
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}
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; Second `add` operand is not `-1` constant
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define i8 @bad_add0(i8 %x, i8 %addop2) {
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; CHECK-LABEL: @bad_add0(
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; CHECK-NEXT: [[RET:%.*]] = add nuw i8 [[X:%.*]], [[ADDOP2:%.*]]
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%ret = add nuw i8 %x, %addop2
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ret i8 %ret
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}
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; Bad constant
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define i8 @bad_add1(i8 %x) {
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; CHECK-LABEL: @bad_add1(
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; CHECK-NEXT: [[RET:%.*]] = add nuw i8 [[X:%.*]], 1
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%ret = add nuw i8 %x, 1 ; not -1
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ret i8 %ret
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}
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define <2 x i8> @bad_add_vec_nonsplat(<2 x i8> %x) {
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; CHECK-LABEL: @bad_add_vec_nonsplat(
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; CHECK-NEXT: [[RET:%.*]] = add nuw <2 x i8> [[X:%.*]], <i8 -1, i8 1>
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; CHECK-NEXT: ret <2 x i8> [[RET]]
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;
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%ret = add nuw <2 x i8> %x, <i8 -1, i8 1>
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ret <2 x i8> %ret
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}
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; Bad known bits
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define i8 @bad_knownbits(i8 %x, i8 %y) {
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; CHECK-LABEL: @bad_knownbits(
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; CHECK-NEXT: [[CMP:%.*]] = icmp slt i8 [[X:%.*]], -3
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; CHECK-NEXT: tail call void @llvm.assume(i1 [[CMP]])
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; CHECK-NEXT: [[RET:%.*]] = add nuw i8 [[X]], [[Y:%.*]]
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; CHECK-NEXT: ret i8 [[RET]]
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;
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%cmp = icmp slt i8 %x, 253
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tail call void @llvm.assume(i1 %cmp)
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%ret = add nuw i8 %x, %y
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ret i8 %ret
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}
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