[Bitcode] Fix constexpr expansion creating invalid PHIs (#141560)

Fixes errors about duplicate PHI edges when the input had duplicates
with constexprs in them. The constexpr translation makes new basic
blocks, causing the verifier to complain about duplicate entries in PHI
nodes.

Author: timower

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6095,14 +6095,18 @@ 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()) {
-          PN->addIncoming(It->second, BB);
+          // If this predecessor was also replaced with a constexpr basic
+          // block, it must be de-duplicated.
+          if (!EdgeBB) {
+            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).

llvm/test/Bitcode/constexpr-to-instr-dups.ll

@@ -0,0 +1,30 @@
+; RUN: opt -expand-constant-exprs %s.bc -S | FileCheck %s
+@foo = external constant i32
+
+define i32 @test(i32 %arg) {
+entry:
+  switch i32 %arg, label %cont [
+    i32 1, label %cont
+    i32 2, label %nonconst
+  ]
+
+nonconst:
+  %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:    %res = phi i32 [ %constexpr1, %phi.constexpr ], [ 1, %nonconst ], [ 1, %nonconst ]
+; CHECK-NEXT:    ret i32 %res
+cont:
+  %res = phi i32 [or (i32 5, i32 ptrtoint (ptr @foo to i32)), %entry],
+                 [or (i32 5, i32 ptrtoint (ptr @foo to i32)), %entry],
+                 [1, %nonconst],
+                 [1, %nonconst]
+  ret i32 %res
+}

llvm/test/Bitcode/constexpr-to-instr-dups.ll.bc

@@ -1,4 +1,4 @@
-; RUN: llvm-dis -expand-constant-exprs < %s.bc | FileCheck %s
+; RUN: opt -expand-constant-exprs -S %s.bc | FileCheck %s
 
 @g = extern_weak global i32
 @g2 = extern_weak global i32
@@ -225,7 +225,7 @@ define i64 @test_phi_multiple_identical_predecessors(i32 %x) {
 ; CHECK-NEXT: %constexpr = ptrtoint ptr @g to i64
 ; CHECK-NEXT: br label %join
 ; CHECK: join:
-; CHECK-NEXT: %phi = phi i64 [ %constexpr, %phi.constexpr ], [ %constexpr, %phi.constexpr ], [ 0, %default ]
+; CHECK-NEXT: %phi = phi i64 [ %constexpr, %phi.constexpr ], [ 0, %default ]
 ; CHECK-NEXT: ret i64 %phi
 entry:
   switch i32 %x, label %default [