This is the first patch to put the needed bits in place to eventually allow code

to be aligned with optimal nops.  This patch does not change any functionality
and when the compiler is changed to use EmitCodeAlignment() it should also not
change the resulting output.  Once the compiler change is made and everything
looks good the next patch with the table of optimal X86 nops will be added to
WriteNopData() changing the output.  There are many FIXMEs in this patch which
will be removed when we have better target hooks (coming soon I hear).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96963 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 528c5c025d..2c698e76f9 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -42,6 +42,8 @@ STATISTIC(EmittedFragments, "Number of emitted assembler fragments");
 static void WriteFileData(raw_ostream &OS, const MCSectionData &SD,
                           MachObjectWriter &MOW);
 
+static uint64_t WriteNopData(uint64_t Count, MachObjectWriter &MOW);
+
 /// isVirtualSection - Check if this is a section which does not actually exist
 /// in the object file.
 static bool isVirtualSection(const MCSection &Section) {
@@ -1058,6 +1060,19 @@ void MCAssembler::LayoutSection(MCSectionData &SD) {
     SD.setFileSize(Address - SD.getAddress());
 }
 
+/// WriteNopData - Write optimal nops to the output file for the \arg Count
+/// bytes.  This returns the number of bytes written.  It may return 0 if
+/// the \arg Count is more than the maximum optimal nops.
+///
+/// FIXME this is X86 32-bit specific and should move to a better place.
+static uint64_t WriteNopData(uint64_t Count, MachObjectWriter &MOW) {
+  // FIXME for now just use the 0x90 nop opcode byte.
+  for (uint64_t i = 0; i < Count; i++)
+    MOW.Write8 (uint8_t(0x90));
+
+  return Count;
+}
+
 /// WriteFileData - Write the \arg F data to the output file.
 static void WriteFileData(raw_ostream &OS, const MCFragment &F,
                           MachObjectWriter &MOW) {
@@ -1081,6 +1096,14 @@ static void WriteFileData(raw_ostream &OS, const MCFragment &F,
                         "' is not a divisor of padding size '" +
                         Twine(AF.getFileSize()) + "'");
 
+    // See if we are aligning with nops, and if so do that first to try to fill
+    // the Count bytes.  Then if that did not fill any bytes or there are any
+    // bytes left to fill use the the Value and ValueSize to fill the rest.
+    if (AF.getEmitNops()) {
+      uint64_t NopByteCount = WriteNopData(Count, MOW);
+      Count -= NopByteCount;
+    }
+
     for (uint64_t i = 0; i != Count; ++i) {
       switch (AF.getValueSize()) {
       default: