From ebc7c74f2f54a3cb6cf86ab26e1296f8311f11e9 Mon Sep 17 00:00:00 2001
From: Nico Weber <nicolasweber@gmx.de>
Date: Tue, 24 Apr 2018 17:29:05 +0000
Subject: [PATCH] Let TableGen write output only if it changed, instead of
 doing so in cmake.

Removes one subprocess and one temp file from the build for each tablegen
invocation.

No intended behavior change.

https://reviews.llvm.org/D45899

llvm-svn: 330742
---
 llvm/cmake/modules/TableGen.cmake | 20 ++++----------------
 llvm/lib/TableGen/Main.cpp        | 21 ++++++++++++++++-----
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/llvm/cmake/modules/TableGen.cmake b/llvm/cmake/modules/TableGen.cmake
index d1afcb4..ee1840d 100644
--- a/llvm/cmake/modules/TableGen.cmake
+++ b/llvm/cmake/modules/TableGen.cmake
@@ -25,7 +25,7 @@ function(tablegen project ofn)
     file(RELATIVE_PATH ofn_rel
       ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR}/${ofn})
     set(additional_cmdline
-      -o ${ofn_rel}.tmp
+      -o ${ofn_rel}
       -d ${ofn_rel}.d
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
       DEPFILE ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.d
@@ -36,7 +36,7 @@ function(tablegen project ofn)
     file(GLOB local_tds "*.td")
     file(GLOB_RECURSE global_tds "${LLVM_MAIN_INCLUDE_DIR}/llvm/*.td")
     set(additional_cmdline
-      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
       )
   endif()
 
@@ -69,8 +69,7 @@ function(tablegen project ofn)
   # dependency twice in the result file when
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
-  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-    # Generate tablegen output in a temporary file.
+  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
@@ -83,20 +82,9 @@ function(tablegen project ofn)
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     COMMENT "Building ${ofn}..."
     )
-  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
-    # Only update the real output file if there are any differences.
-    # This prevents recompilation of all the files depending on it if there
-    # aren't any.
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different
-        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
-    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-    COMMENT "Updating ${ofn}..."
-    )
 
   # `make clean' must remove all those generated files:
-  set_property(DIRECTORY APPEND
-    PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn}.tmp ${ofn})
+  set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn})
 
   set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT} ${CMAKE_CURRENT_BINARY_DIR}/${ofn} PARENT_SCOPE)
   set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/${ofn} PROPERTIES
diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
index be35f89..d230300 100644
--- a/llvm/lib/TableGen/Main.cpp
+++ b/llvm/lib/TableGen/Main.cpp
@@ -96,8 +96,21 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
   if (Parser.ParseFile())
     return 1;
 
+  // Write output to memory.
+  std::string OutString;
+  raw_string_ostream Out(OutString);
+  if (MainFn(Out, Records))
+    return 1;
+
+  // Only updates the real output file if there are any differences.
+  // This prevents recompilation of all the files depending on it if there
+  // aren't any.
+  if (auto ExistingOrErr = MemoryBuffer::getFile(OutputFilename))
+    if (std::move(ExistingOrErr.get())->getBuffer() == Out.str())
+      return 0;
+
   std::error_code EC;
-  ToolOutputFile Out(OutputFilename, EC, sys::fs::F_Text);
+  ToolOutputFile OutFile(OutputFilename, EC, sys::fs::F_Text);
   if (EC)
     return reportError(argv0, "error opening " + OutputFilename + ":" +
                                   EC.message() + "\n");
@@ -105,14 +118,12 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
     if (int Ret = createDependencyFile(Parser, argv0))
       return Ret;
   }
-
-  if (MainFn(Out.os(), Records))
-    return 1;
+  OutFile.os() << Out.str();
 
   if (ErrorsPrinted > 0)
     return reportError(argv0, Twine(ErrorsPrinted) + " errors.\n");
 
   // Declare success.
-  Out.keep();
+  OutFile.keep();
   return 0;
 }
-- 
2.7.4