[GVN] Patch for invalid GVN replacement

If PRE is performed as part of the main GVN pass (to PRE GEP
operands before processing loads), and it is performed across a
backedge, we will end up adding the new instruction to the leader
table of a block that has not yet been processed. When it will be
processed, GVN will incorrectly assume that the value is already
available, even though it is only available at the end of the
block.

Avoid this by not performing PRE across backedges.

Fixes llvm#58418.

Differential Revision: https://reviews.llvm.org/D136095

Author: Alex Gatea

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -2828,17 +2828,10 @@ bool GVNPass::performScalarPRE(Instruction *CurInst) {
       NumWithout = 2;
       break;
     }
-    // It is not safe to do PRE when P->CurrentBlock is a loop backedge, and
-    // when CurInst has operand defined in CurrentBlock (so it may be defined
-    // by phi in the loop header).
+    // It is not safe to do PRE when P->CurrentBlock is a loop backedge.
     assert(BlockRPONumber.count(P) && BlockRPONumber.count(CurrentBlock) &&
            "Invalid BlockRPONumber map.");
-    if (BlockRPONumber[P] >= BlockRPONumber[CurrentBlock] &&
-        llvm::any_of(CurInst->operands(), [&](const Use &U) {
-          if (auto *Inst = dyn_cast<Instruction>(U.get()))
-            return Inst->getParent() == CurrentBlock;
-          return false;
-        })) {
+    if (BlockRPONumber[P] >= BlockRPONumber[CurrentBlock]) {
       NumWithout = 2;
       break;
     }

llvm/test/Transforms/GVN/PRE/load-pre-across-backedge.ll

@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -gvn -S < %s | FileCheck %s
+
+; Check that PRE-LOAD across backedge does not
+; result in invalid dominator tree.
+declare void @use(i32)
+
+define void @test1(i1 %c, i32 %arg) {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[BB1:%.*]], label [[DOTBB2_CRIT_EDGE:%.*]]
+; CHECK:       .bb2_crit_edge:
+; CHECK-NEXT:    [[DOTPRE:%.*]] = shl i32 [[ARG:%.*]], 2
+; CHECK-NEXT:    br label [[BB2:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    [[SHL1:%.*]] = shl i32 [[ARG]], 2
+; CHECK-NEXT:    br label [[BB3:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    [[SHL2_PRE_PHI:%.*]] = phi i32 [ [[DOTPRE]], [[DOTBB2_CRIT_EDGE]] ], [ [[SHL3:%.*]], [[BB3]] ]
+; CHECK-NEXT:    call void @use(i32 [[SHL2_PRE_PHI]])
+; CHECK-NEXT:    br label [[BB3]]
+; CHECK:       bb3:
+; CHECK-NEXT:    [[SHL3]] = shl i32 [[ARG]], 2
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, ptr null, i32 [[SHL3]]
+; CHECK-NEXT:    [[V:%.*]] = load i32, ptr [[GEP]], align 4
+; CHECK-NEXT:    call void @use(i32 [[V]])
+; CHECK-NEXT:    br label [[BB2]]
+;
+  br i1 %c, label %bb1, label %bb2
+
+bb1:
+  %shl1 = shl i32 %arg, 2
+  br label %bb3
+
+bb2:
+  %shl2 = shl i32 %arg, 2
+  call void @use(i32 %shl2)
+  br label %bb3
+
+bb3:
+  %shl3 = shl i32 %arg, 2
+  %gep = getelementptr i32, ptr null, i32 %shl3
+  %v = load i32, ptr %gep, align 4
+  call void @use(i32 %v)
+  br label %bb2
+}

llvm/test/Transforms/GVN/PRE/pre-gep-load.ll

@@ -95,14 +95,13 @@ define void @test_shortcut_safe(i1 %tst, i32 %p1, i32* %a) {
 ; CHECK-LABEL: @test_shortcut_safe(
 ; CHECK-NEXT:    br i1 [[TST:%.*]], label [[SEXT1:%.*]], label [[PRE_DEST:%.*]]
 ; CHECK:       pre.dest:
-; CHECK-NEXT:    [[DOTPRE:%.*]] = sext i32 [[P1:%.*]] to i64
 ; CHECK-NEXT:    br label [[SEXT_USE:%.*]]
 ; CHECK:       sext1:
-; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[P1]] to i64
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[P1:%.*]] to i64
 ; CHECK-NEXT:    br label [[SEXT_USE]]
 ; CHECK:       sext.use:
-; CHECK-NEXT:    [[IDXPROM2_PRE_PHI:%.*]] = phi i64 [ [[IDXPROM]], [[SEXT1]] ], [ [[DOTPRE]], [[PRE_DEST]] ]
-; CHECK-NEXT:    [[ARRAYIDX3:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[IDXPROM2_PRE_PHI]]
+; CHECK-NEXT:    [[IDXPROM2:%.*]] = sext i32 [[P1]] to i64
+; CHECK-NEXT:    [[ARRAYIDX3:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[IDXPROM2]]
 ; CHECK-NEXT:    [[VAL:%.*]] = load i32, i32* [[ARRAYIDX3]], align 4
 ; CHECK-NEXT:    tail call void @g(i32 [[VAL]])
 ; CHECK-NEXT:    br label [[PRE_DEST]]