From 66c63456e206ffa9bda60efe7b168a391ad55216 Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tatham@arm.com>
Date: Tue, 10 Dec 2019 10:34:40 +0000
Subject: [PATCH] [TableGen] Permit dag operators to be unset.

This is not a new semantic feature. The syntax `(? 1, 2, 3)` was
disallowed by the parser in a dag //expression//, but there were
already ways to sneak a `?` into the operator field of a dag
//value//, e.g. by initializing it from a class template parameter
which is then set to `?` by the instantiating `def`.

This patch makes `?` in the operator slot syntactically legal, so it's
now easy to construct dags with an unset operator. Also, the semantics
of `!con` are relaxed so that it will allow a combination of set and
unset operator fields in the dag nodes it's concatenating, with the
restriction that all the operators that are //not// unset still have
to agree with each other.

Reviewers: hfinkel, nhaehnle

Reviewed By: hfinkel, nhaehnle

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71195
---
 lib/TableGen/Record.cpp   | 11 ++++++++---
 lib/TableGen/TGParser.cpp |  3 ++-
 test/TableGen/unsetop.td  | 24 ++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 4 deletions(-)
 create mode 100644 test/TableGen/unsetop.td

diff --git a/lib/TableGen/Record.cpp b/lib/TableGen/Record.cpp
index 2ab7b98ca03..9405fb5ae4e 100644
--- a/lib/TableGen/Record.cpp
+++ b/lib/TableGen/Record.cpp
@@ -887,13 +887,18 @@ Init *BinOpInit::Fold(Record *CurRec) const {
     if (LHSs && RHSs) {
       DefInit *LOp = dyn_cast<DefInit>(LHSs->getOperator());
       DefInit *ROp = dyn_cast<DefInit>(RHSs->getOperator());
-      if (!LOp || !ROp)
+      if ((!LOp && !isa<UnsetInit>(LHSs->getOperator())) ||
+          (!ROp && !isa<UnsetInit>(RHSs->getOperator())))
         break;
-      if (LOp->getDef() != ROp->getDef()) {
+      if (LOp && ROp && LOp->getDef() != ROp->getDef()) {
         PrintFatalError(Twine("Concatenated Dag operators do not match: '") +
                         LHSs->getAsString() + "' vs. '" + RHSs->getAsString() +
                         "'");
       }
+      Init *Op = LOp ? LOp : ROp;
+      if (!Op)
+        Op = UnsetInit::get();
+
       SmallVector<Init*, 8> Args;
       SmallVector<StringInit*, 8> ArgNames;
       for (unsigned i = 0, e = LHSs->getNumArgs(); i != e; ++i) {
@@ -904,7 +909,7 @@ Init *BinOpInit::Fold(Record *CurRec) const {
         Args.push_back(RHSs->getArg(i));
         ArgNames.push_back(RHSs->getArgName(i));
       }
-      return DagInit::get(LHSs->getOperator(), nullptr, Args, ArgNames);
+      return DagInit::get(Op, nullptr, Args, ArgNames);
     }
     break;
   }
diff --git a/lib/TableGen/TGParser.cpp b/lib/TableGen/TGParser.cpp
index c373e2899a5..2ac950b0b6f 100644
--- a/lib/TableGen/TGParser.cpp
+++ b/lib/TableGen/TGParser.cpp
@@ -2024,7 +2024,8 @@ Init *TGParser::ParseSimpleValue(Record *CurRec, RecTy *ItemType,
   }
   case tgtok::l_paren: {         // Value ::= '(' IDValue DagArgList ')'
     Lex.Lex();   // eat the '('
-    if (Lex.getCode() != tgtok::Id && Lex.getCode() != tgtok::XCast) {
+    if (Lex.getCode() != tgtok::Id && Lex.getCode() != tgtok::XCast &&
+        Lex.getCode() != tgtok::question) {
       TokError("expected identifier in dag init");
       return nullptr;
     }
diff --git a/test/TableGen/unsetop.td b/test/TableGen/unsetop.td
new file mode 100644
index 00000000000..7a4f98aea45
--- /dev/null
+++ b/test/TableGen/unsetop.td
@@ -0,0 +1,24 @@
+// RUN: llvm-tblgen %s | FileCheck %s
+// RUN: not llvm-tblgen -DERROR %s 2>&1 | FileCheck --check-prefix=ERROR %s
+
+def op;
+def otherop;
+
+def test {
+  // CHECK: dag d = (? "hello":$world);
+  dag d = (? "hello":$world);
+
+  // CHECK: dag undefNeither = (op 1, 2);
+  dag undefNeither = !con((op 1), (op 2));
+  // CHECK: dag undefFirst = (op 1, 2);
+  dag undefFirst   = !con((?  1), (op 2));
+  // CHECK: dag undefSecond = (op 1, 2);
+  dag undefSecond  = !con((op 1), (?  2));
+  // CHECK: dag undefBoth = (? 1, 2);
+  dag undefBoth    = !con((?  1), (?  2));
+
+#ifdef ERROR
+  // ERROR: Concatenated Dag operators do not match: '(op 1)' vs. '(otherop 2)'
+  dag mismatch     = !con((op 1), (otherop 2));
+#endif
+}