This is a large patch for X86 AVX-512 of an optimization for reducing code size by encoding EVEX AVX-512 instructions using the shorter VEX encoding when possible.

There are cases of AVX-512 instructions that have two possible encodings. This is the case with instructions that use vector registers with low indexes of 0 - 15 and do not use the zmm registers or the mask k registers.
The EVEX encoding prefix requires 4 bytes whereas the VEX prefix can take only up to 3 bytes. Consequently, using the VEX encoding for these instructions results in a code size reduction of ~2 bytes even though it is compiled with the AVX-512 features enabled.

Reviewers: Craig Topper, Zvi Rackoover, Elena Demikhovsky 
Differential Revision: https://reviews.llvm.org/D27901

llvm-svn: 290663

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index 49d1dbf5c06..2f69df064e7 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -16,6 +16,7 @@
 #include "X86RegisterInfo.h"
 #include "X86ShuffleDecodeConstantPool.h"
 #include "InstPrinter/X86ATTInstPrinter.h"
+#include "InstPrinter/X86InstComments.h"
 #include "MCTargetDesc/X86BaseInfo.h"
 #include "Utils/X86ShuffleDecode.h"
 #include "llvm/ADT/Optional.h"
@@ -1290,6 +1291,13 @@ void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
   X86MCInstLower MCInstLowering(*MF, *this);
   const X86RegisterInfo *RI = MF->getSubtarget<X86Subtarget>().getRegisterInfo();
 
+  // Add a comment about EVEX-2-VEX compression for AVX-512 instrs that
+  // are compressed from EVEX encoding to VEX encoding.
+  if (TM.Options.MCOptions.ShowMCEncoding) {
+    if (MI->getAsmPrinterFlags() & AC_EVEX_2_VEX)
+      OutStreamer->AddComment("EVEX TO VEX Compression ", false);
+  }
+
   switch (MI->getOpcode()) {
   case TargetOpcode::DBG_VALUE:
     llvm_unreachable("Should be handled target independently");