JIT: fold trees after inline return expression updates (#1751)

Aggressively fold as we substitute inline return value trees in for the return
value placeholders. Notice when this folding leads to branch simplification,
and make the associated flow graph update.

The recently added early flow opt pass will then transitively remove any
newly unreachable code.

Resolves dotnet/coreclr#27395.

src/coreclr/src/jit/flowgraph.cpp

@@ -22315,7 +22315,7 @@ Compiler::fgWalkResult Compiler::fgUpdateInlineReturnExpressionPlaceHolder(GenTr
  Compiler* comp = data->compiler;
  CORINFO_CLASS_HANDLE retClsHnd = NO_CLASS_HANDLE;
 
- if (tree->OperGet() == GT_RET_EXPR)
+ while (tree->OperGet() == GT_RET_EXPR)
  {
  // We are going to copy the tree from the inlinee,
  // so record the handle now.
@@ -22327,8 +22327,15 @@ Compiler::fgWalkResult Compiler::fgUpdateInlineReturnExpressionPlaceHolder(GenTr
 
  // Skip through chains of GT_RET_EXPRs (say from nested inlines)
  // to the actual tree to use.
- GenTree* inlineCandidate = tree->gtRetExprVal();
- var_types retType = tree->TypeGet();
+ //
+ // Also we might as well try and fold the return value.
+ // Eg returns of constant bools will have CASTS.
+ // This folding may uncover more GT_RET_EXPRs, so we loop around
+ // until we've got something distinct.
+ //
+ GenTree* inlineCandidate = tree->gtRetExprVal();
+ inlineCandidate = comp->gtFoldExpr(inlineCandidate);
+ var_types retType = tree->TypeGet();
 
 #ifdef DEBUG
  if (comp->verbose)
@@ -22598,6 +22605,64 @@ Compiler::fgWalkResult Compiler::fgLateDevirtualization(GenTree** pTree, fgWalkD
  }
  }
  }
+ else if (tree->OperGet() == GT_JTRUE)
+ {
+ // See if this jtrue is now foldable.
+ BasicBlock* block = comp->compCurBB;
+ GenTree* condTree = tree->AsOp()->gtOp1;
+ assert(tree == block->lastStmt()->GetRootNode());
+
+ if (condTree->OperGet() == GT_CNS_INT)
+ {
+ JITDUMP(" ... found foldable jtrue at [%06u] in BB%02u\n", dspTreeID(tree), block->bbNum);
+ noway_assert((block->bbNext->countOfInEdges() > 0) && (block->bbJumpDest->countOfInEdges() > 0));
+
+ // It would be nice to assert that "tree" is side-effect
+ // free, before we bash it.
+ //
+ // But we expect to see the GTF_CALL flag set, because
+ // this tree is an ancestor of an inline return
+ // value placeholder, and those always have GTF_CALL set,
+ // since if inlining fails we'd swap the call back in
+ // place.
+ //
+ // So, check that at least everything else is clear.
+ assert((tree->gtFlags & (GTF_SIDE_EFFECT & ~GTF_CALL)) == 0);
+
+ tree->gtBashToNOP();
+
+ BasicBlock* bTaken = nullptr;
+ BasicBlock* bNotTaken = nullptr;
+
+ if (condTree->AsIntCon()->gtIconVal != 0)
+ {
+ block->bbJumpKind = BBJ_ALWAYS;
+ bTaken = block->bbJumpDest;
+ bNotTaken = block->bbNext;
+ }
+ else
+ {
+ block->bbJumpKind = BBJ_NONE;
+ bTaken = block->bbNext;
+ bNotTaken = block->bbJumpDest;
+ }
+
+ comp->fgRemoveRefPred(bNotTaken, block);
+
+ // If that was the last ref, a subsequent flow-opt pass
+ // will clean up the now-unreachable bNotTaken, and any
+ // other transitively unreachable blocks.
+ if (bNotTaken->bbRefs == 0)
+ {
+ JITDUMP("... it looks like BB%02u is now unreachable!\n", bNotTaken->bbNum);
+ }
+ }
+ }
+ else
+ {
+ GenTree* foldedTree = comp->gtFoldExpr(tree);
+ *pTree = foldedTree;
+ }
 
  return WALK_CONTINUE;
 }