[PATCH] D75799: Don't create PHI nodes with "is.constant" values

[Bill Wendling via Phabricator via llvm-commits]

void updated this revision to Diff 248920.
void added a comment.

Use pattern matching for intrinsic instruction matching.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D75799/new/

https://reviews.llvm.org/D75799

Files:
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/is_constant.ll


Index: llvm/test/Transforms/JumpThreading/is_constant.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/is_constant.ll
@@ -0,0 +1,39 @@
+; RUN: opt -jump-threading -S -verify < %s | FileCheck %s
+
+; CHECK-LABEL: define void @test1(
+; CHECK-LABEL: if.end18:
+; CHECK-NOT:   call i1 @llvm.is.constant.i64
+define void @test1() {
+entry:
+  %conv = trunc i64 undef to i32
+  %conv8 = ashr exact i64 undef, 32
+  %cmp9 = icmp ugt i64 %conv8, 24
+  br i1 %cmp9, label %if.then11, label %if.end18
+
+if.then11:                                        ; preds = %entry
+  br label %if.end18
+
+if.end18:                                         ; preds = %if.then11, %entry
+  %len.0 = phi i32 [ 24, %if.then11 ], [ %conv, %entry ]
+  %cmp3.i.i70 = icmp ugt i32 %len.0, 24
+  %0 = call i1 @llvm.is.constant.i64(i64 undef) #1
+  br i1 %cmp3.i.i70, label %if.then.i.i71, label %if.end12.i.i74
+
+if.then.i.i71:                                    ; preds = %if.end18
+  unreachable
+
+if.end12.i.i74:                                   ; preds = %if.end18
+  br i1 %0, label %if.else.i.i, label %if.then7.i.i
+
+if.then7.i.i:                                     ; preds = %if.end12.i.i74
+  ret void
+
+if.else.i.i:                                      ; preds = %if.end12.i.i74
+  unreachable
+}
+
+; Function Attrs: nounwind readnone willreturn
+declare i1 @llvm.is.constant.i64(i64) #0
+
+attributes #0 = { nounwind readnone willreturn }
+attributes #1 = { nounwind }
Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -2003,6 +2003,39 @@
 void JumpThreadingPass::UpdateSSA(
     BasicBlock *BB, BasicBlock *NewBB,
     DenseMap<Instruction *, Value *> &ValueMapping) {
+  // Don't create PHI nodes with values from is_constant intrinsics. Code gated
+  // by these intrinsics sometimes rely upon their evaluation to DCE invalid
+  // paths. Combining them into PHI nodes complicates the evaluation.
+  SmallVector<Instruction *, 2> IsConstantIntrs;
+  for (Instruction &I : *BB) {
+    using namespace PatternMatch;
+    if (match(&I, m_Intrinsic<Intrinsic::is_constant>()))
+      continue;
+
+    // Scan all uses of this instruction to see if it is used outside of its
+    // block.
+    for (Use &U : I.uses()) {
+      Instruction *User = cast<Instruction>(U.getUser());
+      if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
+        if (UserPN->getIncomingBlock(U) == BB)
+          continue;
+      } else if (User->getParent() == BB) {
+        continue;
+      }
+
+      IsConstantIntrs.push_back(&I);
+      break;
+    }
+  }
+
+  for (auto *II : IsConstantIntrs) {
+    Constant *Res = isa<Constant>(II->getOperand(0))
+                        ? ConstantInt::getTrue(II->getType())
+                        : ConstantInt::getFalse(II->getType());
+    II->replaceAllUsesWith(Res);
+    II->eraseFromParent();
+  }
+
   // If there were values defined in BB that are used outside the block, then we
   // now have to update all uses of the value to use either the original value,
   // the cloned value, or some PHI derived value.  This can require arbitrary
@@ -2331,8 +2364,8 @@
 
   // And finally, do it!
   LLVM_DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName()
-                    << "' to '" << SuccBB->getName()
-                    << ", across block:\n    " << *BB << "\n");
+                    << "' to '" << SuccBB->getName() << "', across block:\n    "
+                    << *BB << "\n");
 
   if (DTU->hasPendingDomTreeUpdates())
     LVI->disableDT();


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