[PATCH] D77160: Fix PR45371: SeparateConstOffsetFromGEP clean up bookkeeping

[Jonathan Roelofs via Phabricator via llvm-commits]

jroelofs created this revision.
jroelofs added reviewers: uabelho, jingyue.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

find() was altering the UserChain, even in cases where it subsequently
discovered that the resulting constant was a 0. This confuses
rebuildWithoutConstOffset() when it attempts to walk the chain later, since it
is expected that the chain itself be a path down the use-def edges of an
expression.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D77160

Files:
  llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
  llvm/test/Transforms/SeparateConstOffsetFromGEP/pr45371.ll


Index: llvm/test/Transforms/SeparateConstOffsetFromGEP/pr45371.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SeparateConstOffsetFromGEP/pr45371.ll
@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -separate-const-offset-from-gep < %s | FileCheck %s
+
+ at e = external global [4000 x i8], align 1
+
+define void @PR45371() {
+; CHECK-LABEL: @PR45371(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 65536, undef
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i32 [[SUB]] to i8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[TMP0]], 96
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i8 0 to i64
+; CHECK-NEXT:    [[IDXPROM1:%.*]] = sext i8 [[TMP1]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [4000 x i8], [4000 x i8]* @e, i64 [[IDXPROM]], i64 [[IDXPROM1]]
+; CHECK-NEXT:    ret void
+;
+entry:
+  %sub = sub nsw i32 65536, undef
+  %0 = trunc i32 %sub to i8
+  %1 = add i8 %0, -4000
+  %arrayidx = getelementptr inbounds [4000 x i8], [4000 x i8]* @e, i8 0, i8 %1
+  ret void
+}
Index: llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
+++ llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
@@ -522,7 +522,7 @@
     //   sext(a + b) = sext(a) + sext(b)
     // even if the addition is not marked nsw.
     //
-    // Leveraging this invarient, we can trace into an sext'ed inbound GEP
+    // Leveraging this invariant, we can trace into an sext'ed inbound GEP
     // index if the constant offset is non-negative.
     //
     // Verified in @sext_add in split-gep.ll.
@@ -552,6 +552,9 @@
 APInt ConstantOffsetExtractor::findInEitherOperand(BinaryOperator *BO,
                                                    bool SignExtended,
                                                    bool ZeroExtended) {
+  // Save off the current height of the chain, in case we need to restore it.
+  size_t ChainLength = UserChain.size();
+
   // BO being non-negative does not shed light on whether its operands are
   // non-negative. Clear the NonNegative flag here.
   APInt ConstantOffset = find(BO->getOperand(0), SignExtended, ZeroExtended,
@@ -562,12 +565,23 @@
   // However, such cases are probably already handled by -instcombine,
   // given this pass runs after the standard optimizations.
   if (ConstantOffset != 0) return ConstantOffset;
+
+  // Reset the chain back to where it was when we started exploring this node,
+  // since visiting the LHS didn't pan out.
+  UserChain.resize(ChainLength);
+
   ConstantOffset = find(BO->getOperand(1), SignExtended, ZeroExtended,
                         /* NonNegative */ false);
   // If U is a sub operator, negate the constant offset found in the right
   // operand.
   if (BO->getOpcode() == Instruction::Sub)
     ConstantOffset = -ConstantOffset;
+
+  if (ConstantOffset == 0) {
+    // Reset the chain again, since exploring the RHS didn't work out either.
+    UserChain.resize(ChainLength);
+  }
+
   return ConstantOffset;
 }
 


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