[llvm-elfabi] Add flag to preserve timestamp when output is the same

This change adds '--write-if-changed' flag to llvm-elfabi tool. When
enabled, llvm-elfabi will not overwrite the existing file if the
content of the file will not be changed, which preserves the
timestamp.

Differential Revision: https://reviews.llvm.org/D92902

1 files changed, 27 insertions, 12 deletions

diff --git a/llvm/lib/InterfaceStub/ELFObjHandler.cpp b/llvm/lib/InterfaceStub/ELFObjHandler.cpp
index e50ebd7..40d8afa 100644
--- a/llvm/lib/InterfaceStub/ELFObjHandler.cpp
+++ b/llvm/lib/InterfaceStub/ELFObjHandler.cpp
@@ -17,6 +17,7 @@
 #include "llvm/Support/FileOutputBuffer.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Process.h"
 
 using llvm::MemoryBufferRef;
 using llvm::object::ELFObjectFile;
@@ -663,8 +664,25 @@ buildStub(const ELFObjectFile<ELFT> &ElfObj) {
 /// @param FilePath File path for writing the ELF binary.
 /// @param Stub Source ELFStub to generate a binary ELF stub from.
 template <class ELFT>
-static Error writeELFBinaryToFile(StringRef FilePath, const ELFStub &Stub) {
+static Error writeELFBinaryToFile(StringRef FilePath, const ELFStub &Stub,
+                                  bool WriteIfChanged) {
   ELFStubBuilder<ELFT> Builder{Stub};
+  // Write Stub to memory first.
+  std::vector<uint8_t> Buf(Builder.getSize());
+  Builder.write(Buf.data());
+
+  if (WriteIfChanged) {
+    if (ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrError =
+            MemoryBuffer::getFile(FilePath)) {
+      // Compare Stub output with existing Stub file.
+      // If Stub file unchanged, abort updating.
+      if ((*BufOrError)->getBufferSize() == Builder.getSize() &&
+          !memcmp((*BufOrError)->getBufferStart(), Buf.data(),
+                  Builder.getSize()))
+        return Error::success();
+    }
+  }
+
   Expected<std::unique_ptr<FileOutputBuffer>> BufOrError =
       FileOutputBuffer::create(FilePath, Builder.getSize());
   if (!BufOrError)
@@ -674,13 +692,10 @@ static Error writeELFBinaryToFile(StringRef FilePath, const ELFStub &Stub) {
                                  "` for writing");
 
   // Write binary to file.
-  std::unique_ptr<FileOutputBuffer> Buf = std::move(*BufOrError);
-  Builder.write(Buf->getBufferStart());
+  std::unique_ptr<FileOutputBuffer> FileBuf = std::move(*BufOrError);
+  memcpy(FileBuf->getBufferStart(), Buf.data(), Buf.size());
 
-  if (Error E = Buf->commit())
-    return E;
-
-  return Error::success();
+  return FileBuf->commit();
 }
 
 Expected<std::unique_ptr<ELFStub>> readELFFile(MemoryBufferRef Buf) {
@@ -705,15 +720,15 @@ Expected<std::unique_ptr<ELFStub>> readELFFile(MemoryBufferRef Buf) {
 // This function wraps the ELFT writeELFBinaryToFile() so writeBinaryStub()
 // can be called without having to use ELFType templates directly.
 Error writeBinaryStub(StringRef FilePath, const ELFStub &Stub,
-                      ELFTarget OutputFormat) {
+                      ELFTarget OutputFormat, bool WriteIfChanged) {
   if (OutputFormat == ELFTarget::ELF32LE)
-    return writeELFBinaryToFile<ELF32LE>(FilePath, Stub);
+    return writeELFBinaryToFile<ELF32LE>(FilePath, Stub, WriteIfChanged);
   if (OutputFormat == ELFTarget::ELF32BE)
-    return writeELFBinaryToFile<ELF32BE>(FilePath, Stub);
+    return writeELFBinaryToFile<ELF32BE>(FilePath, Stub, WriteIfChanged);
   if (OutputFormat == ELFTarget::ELF64LE)
-    return writeELFBinaryToFile<ELF64LE>(FilePath, Stub);
+    return writeELFBinaryToFile<ELF64LE>(FilePath, Stub, WriteIfChanged);
   if (OutputFormat == ELFTarget::ELF64BE)
-    return writeELFBinaryToFile<ELF64BE>(FilePath, Stub);
+    return writeELFBinaryToFile<ELF64BE>(FilePath, Stub, WriteIfChanged);
   llvm_unreachable("invalid binary output target");
 }