[TableGen] Emit the primary input file in -d depfile output (#197061)

rnk · web-flow · commit 05c0db7f0aa1 · 2026-05-12T02:26:50.000Z
This fixes a bug where old, but still supported, versions of CMake and
ninja perpetually consider zero-include tablegen files to be out of
date. It also matches what Clang and GCC do for regular C compilations.

When a .td input has no `include` directives, the depfile produced by
`-d` contains only `&lt;output&gt;:` followed by zero dependencies. My version
(3.27) of CMake's `cmake_transform_depfile` step then writes a 0-byte
file, which old versions of ninja treat as a missing depfile and re-run
the rule on every incremental build (e.g. Attributes.td, ValueTypes.td).

Here's the effect on Attributes.inc.d:

```
$ cat ./build/include/llvm/IR/Attributes.inc.d
Attributes.inc:
# switch branches and rebuild...
$ cat ./build/include/llvm/IR/Attributes.inc.d
Attributes.inc: /work/llvm-project/llvm/include/llvm/IR/Attributes.td
```

An LLM was used to help create this change.
diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
@@ -97,6 +97,16 @@ static int createDependencyFile(const TGParser &Parser, const char *argv0) {
     return reportError(argv0, "error opening " + DependFilename + ":" +
                                   EC.message() + "\n");
   DepOut.os() << OutputFilename << ":";
+
+  // Emit the primary input file as a dependency. This matches C compilers like
+  // Clang and GCC. Without it, a .td file with no `include` directives would
+  // produce a depfile listing zero dependencies. CMake's
+  // `cmake_transform_depfile` then collapses that to a 0-byte file, which Ninja
+  // treats as a missing depfile and re-runs the rule on every incremental
+  // build.
+  if (InputFilename != "-")
+    DepOut.os() << ' ' << InputFilename;
+
   for (const auto &Dep : Parser.getDependencies()) {
     DepOut.os() << ' ' << Dep;
   }
diff --git a/llvm/test/TableGen/depfile.td b/llvm/test/TableGen/depfile.td
@@ -0,0 +1,19 @@
+// Verify that `-d` emits a non-empty depfile that always names the primary
+// input file, even when the input has no `include` directives. A depfile
+// containing only `<output>:` (no deps) round-trips through CMake's
+// `cmake_transform_depfile` step to a 0-byte file, which Ninja treats as a
+// missing depfile and unconditionally re-runs the edge.
+
+// RUN: llvm-tblgen -print-records %s -o %t.out -d %t.d
+// RUN: FileCheck %s --input-file=%t.d
+
+// CHECK: {{.*}}depfile.td.tmp.out: {{.*}}depfile.td
+
+// When the input is stdin (`-`), there is no input path to emit, so the
+// depfile lists only the output target with no dependencies.
+// RUN: llvm-tblgen -print-records - -o %t.stdin.out -d %t.stdin.d < %s
+// RUN: FileCheck %s --check-prefix=STDIN --input-file=%t.stdin.d
+
+// STDIN: {{.*}}depfile.td.tmp.stdin.out:{{$}}
+
+def Empty;
