[LoopSimplifyCFG] Prevent use-def dominance breach by handling dead e…

…xits. PR56243 One of the transforms in LoopSimplifyCFG demands that the LCSSA form is truly maintained for all values, tokens included, otherwise it may end up creating a use that is not dominated by def (and Phi creation for tokens is impossible). Detect this situation and prevent transform for it early. Differential Revision: https://reviews.llvm.org/D129984 Reviewed By: efriedma
llvm · Jul 19, 2022 · 8230983 · 8230983
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Showing 2 changed files with 46 additions and 8 deletions.
diff --git a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -576,6 +576,18 @@ class ConstantTerminatorFoldingImpl {
       return false;
     }
 
+    // TODO: Tokens may breach LCSSA form by default. However, the transform for
+    // dead exit blocks requires LCSSA form to be maintained for all values,
+    // tokens included, otherwise it may break use-def dominance (see PR56243).
+    if (!DeadExitBlocks.empty() && !L.isLCSSAForm(DT, /*IgnoreTokens*/ false)) {
+      assert(L.isLCSSAForm(DT, /*IgnoreTokens*/ true) &&
+             "LCSSA broken not by tokens?");
+      LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
+                        << Header->getName()
+                        << ": tokens uses potentially break LCSSA form.\n");
+      return false;
+    }
+
     SE.forgetTopmostLoop(&L);
     // Dump analysis results.
     LLVM_DEBUG(dump());

diff --git a/llvm/test/Transforms/LoopSimplifyCFG/pr56243.ll b/llvm/test/Transforms/LoopSimplifyCFG/pr56243.ll
@@ -1,25 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -passes='loop-mssa(licm,loop-simplifycfg)' < %s | FileCheck %s
 
-; REQUIRES: asserts
-; XFAIL: *
-
 ; Here we end un sinking a user of token down from the loop, therefore breaching LCSSA form.
 ; Then, LoopSimplifyCFG expcets that LCSSA form is maintained, and remains unaware that
 ; it may be penetrated by tokens. As result, it may end up breaking dominance between def and
 ; use by introducing fake temporary edges.
 
 define i8 addrspace(1)* @test_gc_relocate() gc "statepoint-example" {
-; CHECK-LABEL: @test_gc_relocate
+; CHECK-LABEL: @test_gc_relocate(
+; CHECK-NEXT:    br label [[BB1:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    switch i32 undef, label [[BB43:%.*]] [
+; CHECK-NEXT:    i32 1, label [[BB18:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb18:
+; CHECK-NEXT:    switch i32 undef, label [[BB43]] [
+; CHECK-NEXT:    i32 0, label [[BB28:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb28:
+; CHECK-NEXT:    [[TMP34:%.*]] = call token (i64, i32, i8 addrspace(1)* (i64, i32, i32, i32)*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_p1i8i64i32i32i32f(i64 2882400000, i32 0, i8 addrspace(1)* (i64, i32, i32, i32)* nonnull elementtype(i8 addrspace(1)* (i64, i32, i32, i32)) @barney.4, i32 4, i32 0, i64 undef, i32 5, i32 5, i32 undef, i32 0, i32 0) [ "deopt"(), "gc-live"(i8 addrspace(1)* undef) ]
+; CHECK-NEXT:    br i1 false, label [[BB57:%.*]], label [[BB36:%.*]]
+; CHECK:       bb36:
+; CHECK-NEXT:    switch i32 undef, label [[BB43]] [
+; CHECK-NEXT:    i32 1, label [[BB39:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb39:
+; CHECK-NEXT:    switch i32 undef, label [[BB43]] [
+; CHECK-NEXT:    i32 1, label [[BB45:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb43:
+; CHECK-NEXT:    unreachable
+; CHECK:       bb45:
+; CHECK-NEXT:    br label [[BB1]]
+; CHECK:       bb57:
+; CHECK-NEXT:    [[TMP35_LE:%.*]] = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token [[TMP34]], i32 0, i32 0)
+; CHECK-NEXT:    ret i8 addrspace(1)* [[TMP35_LE]]
+;
   br label %bb1
 
 bb1:                                              ; preds = %bb45, %0
   switch i32 undef, label %bb43 [
-    i32 1, label %bb18
+  i32 1, label %bb18
   ]
 
 bb18:                                             ; preds = %bb1
   switch i32 undef, label %bb43 [
-    i32 0, label %bb28
+  i32 0, label %bb28
   ]
 
 bb28:                                             ; preds = %bb18
@@ -29,12 +55,12 @@ bb28:                                             ; preds = %bb18
 
 bb36:                                             ; preds = %bb28
   switch i32 undef, label %bb43 [
-    i32 1, label %bb39
+  i32 1, label %bb39
   ]
 
 bb39:                                             ; preds = %bb36
   switch i32 undef, label %bb43 [
-    i32 1, label %bb45
+  i32 1, label %bb45
   ]
 
 bb43:                                             ; preds = %bb39, %bb36, %bb18, %bb1