From a1d65a5f38898b3c60bc751ef71e1df7e2a1f4fc Mon Sep 17 00:00:00 2001
From: Florian Stamer <florian.stamer@guardsquare.com>
Date: Fri, 10 Oct 2025 16:15:13 +0200
Subject: [PATCH] [Bitcode] Fix incomplete deduplication of PHI entries

---
 llvm/lib/Bitcode/Reader/BitcodeReader.cpp     |  33 ++++++++++++++----
 llvm/test/Bitcode/constexpr-to-instr-dups.ll  |  27 ++++++++++++++
 .../Bitcode/constexpr-to-instr-dups.ll.bc     | Bin 1436 -> 1572 bytes
 3 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index aaee1f0a7687c..dae7b3966d507 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -4093,6 +4093,30 @@ GlobalValue::SanitizerMetadata deserializeSanitizerMetadata(unsigned V) {
   return Meta;
 }
 
+static void deduplicatePhiUsesOf(BasicBlock *BB, BasicBlock *ToDeduplicateBB) {
+  // N.B. This might not be a complete BasicBlock, so don't assume
+  // that it ends with a non-phi instruction.
+  for (Instruction &I : *BB) {
+    PHINode *PN = dyn_cast<PHINode>(&I);
+    if (!PN)
+      break;
+    // Since the order of basic blocks in a PHINode are not generally sorted we
+    // have to iterate over all indicies.
+    unsigned Idx = 0;
+    bool SkippedFirstOccurrence = false;
+    while (Idx < PN->getNumIncomingValues()) {
+      if (PN->getIncomingBlock(Idx) == ToDeduplicateBB) {
+        if (SkippedFirstOccurrence) {
+          PN->removeIncomingValue(Idx);
+          continue;
+        }
+        SkippedFirstOccurrence = true;
+      }
+      ++Idx;
+    }
+  }
+}
+
 Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
   // v1: [pointer type, isconst, initid, linkage, alignment, section,
   // visibility, threadlocal, unnamed_addr, externally_initialized,
@@ -6105,18 +6129,14 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
         // seen value here, to avoid expanding a constant expression multiple
         // times.
         auto It = Args.find(BB);
-        BasicBlock *EdgeBB = ConstExprEdgeBBs.lookup({BB, CurBB});
         if (It != Args.end()) {
-          // If this predecessor was also replaced with a constexpr basic
-          // block, it must be de-duplicated.
-          if (!EdgeBB) {
-            PN->addIncoming(It->second, BB);
-          }
+          PN->addIncoming(It->second, BB);
           continue;
         }
 
         // If there already is a block for this edge (from a different phi),
         // use it.
+        BasicBlock *EdgeBB = ConstExprEdgeBBs.lookup({BB, CurBB});
         if (!EdgeBB) {
           // Otherwise, use a temporary block (that we will discard if it
           // turns out to be unnecessary).
@@ -6944,6 +6964,7 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
     BranchInst::Create(To, EdgeBB);
     From->getTerminator()->replaceSuccessorWith(To, EdgeBB);
     To->replacePhiUsesWith(From, EdgeBB);
+    deduplicatePhiUsesOf(To, EdgeBB);
     EdgeBB->moveBefore(To);
   }
 
diff --git a/llvm/test/Bitcode/constexpr-to-instr-dups.ll b/llvm/test/Bitcode/constexpr-to-instr-dups.ll
index dee9490b616fa..d439a6dcf0c7b 100644
--- a/llvm/test/Bitcode/constexpr-to-instr-dups.ll
+++ b/llvm/test/Bitcode/constexpr-to-instr-dups.ll
@@ -28,3 +28,30 @@ cont:
                  [1, %nonconst]
   ret i32 %res
 }
+
+define i32 @test_multiple_phis(i32 %arg) {
+entry:
+  switch i32 %arg, label %cont [
+    i32 1, label %cont
+    i32 2, label %nonconst
+  ]
+
+nonconst:                                         ; preds = %entry
+  %cmp = icmp ne i32 %arg, 2
+  br i1 %cmp, label %cont, label %cont
+
+; CHECK-LABEL: phi.constexpr:
+; CHECK-NEXT:    %constexpr = ptrtoint ptr @foo to i32
+; CHECK-NEXT:    %constexpr1 = or i32 %constexpr, 5
+; CHECK-NEXT:    br label %cont
+
+
+; CHECK-LABEL: cont:
+; CHECK-NEXT:    %phi1 = phi i32 [ 0, %phi.constexpr ], [ 1, %nonconst ], [ 1, %nonconst ]
+; CHECK-NEXT:    %phi2 = phi i32 [ %constexpr1, %phi.constexpr ], [ 1, %nonconst ], [ 1, %nonconst ]
+; CHECK-NEXT:    ret i32 %phi2
+cont:                                             ; preds = %nonconst, %nonconst, %entry, %entry
+  %phi1 = phi i32 [ 0, %entry ], [ 0, %entry ], [ 1, %nonconst ], [ 1, %nonconst ]
+  %phi2 = phi i32 [ or (i32 ptrtoint (ptr @foo to i32), i32 5), %entry ], [ or (i32 ptrtoint (ptr @foo to i32), i32 5), %entry ], [ 1, %nonconst ], [ 1, %nonconst ]
+  ret i32 %phi2
+}
diff --git a/llvm/test/Bitcode/constexpr-to-instr-dups.ll.bc b/llvm/test/Bitcode/constexpr-to-instr-dups.ll.bc
index 7897f51322fcc3e9ce738c66fc90d36dde5c1fb1..15a3eeb449d8780f44eedf7707916aa474a7fff3 100644
GIT binary patch
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