[llvm] dd48a9b - [ValueTracking] Handle conflicts when computing knownbits of PHI nodes in deadcode; PR65022
Noah Goldstein via llvm-commits
llvm-commits at lists.llvm.org
Fri Sep 1 00:12:13 PDT 2023
Author: Noah Goldstein
Date: 2023-09-01T02:11:50-05:00
New Revision: dd48a9b0561cc65b8597d618f8b286682866c66d
URL: https://github.com/llvm/llvm-project/commit/dd48a9b0561cc65b8597d618f8b286682866c66d
DIFF: https://github.com/llvm/llvm-project/commit/dd48a9b0561cc65b8597d618f8b286682866c66d.diff
LOG: [ValueTracking] Handle conflicts when computing knownbits of PHI nodes in deadcode; PR65022
Bug was introduced in: https://reviews.llvm.org/D157807
The prior logic assumed that the information from the knownbits
returned from analyzing the `icmp` and its operands in the context
basicblock would be consistent.
This is not necessarily the case if we are analyzing deadcode.
For example with `(icmp sgt (select cond, 0, 1), -1)`. If we take
knownbits for the `select` using knownbits from the operator, we will
know the signbit is zero. If we are analyzing a not-taken from based
on the `icmp` (deadcode), we will also "know" that the `select` must
be negative (signbit is one). This will result in a conflict in
knownbits.
The fix is to just give up on analying the phi-node if its deadcode. We 1) don't want to waste time continuing to analyze it and 2) will be removing it (and its dependencies) later.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D158960
Added:
llvm/test/Analysis/ValueTracking/knownbits-phi-deadcode.ll
Modified:
llvm/lib/Analysis/ValueTracking.cpp
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0ea47f1dffc3ca..801814e0c18a83 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1480,7 +1480,17 @@ static void computeKnownBitsFromOperator(const Operator *I,
Pred = CmpInst::getInversePredicate(Pred);
// Get the knownbits implied by the incoming phi condition.
auto CR = ConstantRange::makeExactICmpRegion(Pred, *RHSC);
- Known2 = Known2.unionWith(CR.toKnownBits());
+ KnownBits KnownUnion = Known2.unionWith(CR.toKnownBits());
+ // We can have conflicts here if we are analyzing deadcode (its
+ // impossible for us reach this BB based the icmp).
+ if (KnownUnion.hasConflict()) {
+ // No reason to continue analyzing in a known dead region, so
+ // just resetAll and break. This will cause us to also exit the
+ // outer loop.
+ Known.resetAll();
+ break;
+ }
+ Known2 = KnownUnion;
}
}
}
diff --git a/llvm/test/Analysis/ValueTracking/knownbits-phi-deadcode.ll b/llvm/test/Analysis/ValueTracking/knownbits-phi-deadcode.ll
new file mode 100644
index 00000000000000..6ccb87471fcb08
--- /dev/null
+++ b/llvm/test/Analysis/ValueTracking/knownbits-phi-deadcode.ll
@@ -0,0 +1,57 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 2
+; RUN: opt -passes=loop-deletion -S < %s | FileCheck %s
+; This reduced testcase from pr65022. We are only testing that is doesn't crash.
+
+define void @f(i32 %spec.select) {
+; CHECK-LABEL: define void @f
+; CHECK-SAME: (i32 [[SPEC_SELECT:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[L_OUTER:%.*]]
+; CHECK: L.outer:
+; CHECK-NEXT: br label [[IF_END:%.*]]
+; CHECK: if.end:
+; CHECK-NEXT: [[DOTB:%.*]] = load i1, ptr null, align 1
+; CHECK-NEXT: [[TMP0:%.*]] = select i1 [[DOTB]], i32 0, i32 1
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[TMP0]], -1
+; CHECK-NEXT: br i1 [[CMP]], label [[IF_THEN3:%.*]], label [[WHILE_COND_SPLIT_LOOP_EXIT1:%.*]]
+; CHECK: if.then3:
+; CHECK-NEXT: br label [[IF_END]]
+; CHECK: while.cond.split.loop.exit1:
+; CHECK-NEXT: [[DOTLCSSA:%.*]] = phi i32 [ [[TMP0]], [[IF_END]] ]
+; CHECK-NEXT: [[NOT_LE:%.*]] = xor i32 [[DOTLCSSA]], 1
+; CHECK-NEXT: [[TOBOOL7_NOT:%.*]] = icmp eq i32 [[NOT_LE]], 0
+; CHECK-NEXT: [[SPEC_SELECT3:%.*]] = select i1 [[TOBOOL7_NOT]], i32 0, i32 [[SPEC_SELECT]]
+; CHECK-NEXT: [[TOBOOL10_NOT:%.*]] = icmp eq i32 [[SPEC_SELECT3]], 0
+; CHECK-NEXT: br i1 [[TOBOOL10_NOT]], label [[FOR_COND_PREHEADER:%.*]], label [[L_OUTER]]
+; CHECK: for.cond.preheader:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %L.outer
+
+L.outer: ; preds = %while.cond.split.loop.exit1, %entry
+ br label %if.end
+
+if.end: ; preds = %if.then3, %L.outer
+ %.b = load i1, ptr null, align 1
+ %0 = select i1 %.b, i32 0, i32 1
+ %cmp = icmp sgt i32 %0, -1
+ br i1 %cmp, label %if.then3, label %while.cond.split.loop.exit1
+
+if.then3: ; preds = %if.end
+ br label %if.end
+
+while.cond.split.loop.exit1: ; preds = %if.end
+ %.lcssa = phi i32 [ %0, %if.end ]
+ %not.le = xor i32 %.lcssa, 1
+ %tobool7.not = icmp eq i32 %not.le, 0
+ %spec.select3 = select i1 %tobool7.not, i32 0, i32 %spec.select
+ %tobool10.not = icmp eq i32 %spec.select3, 0
+ br i1 %tobool10.not, label %for.cond.preheader, label %L.outer
+
+for.cond.preheader: ; preds = %while.cond.split.loop.exit1
+ ret void
+}
+
+; uselistorder directives
+uselistorder i32 0, { 1, 0, 2, 3 }
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