[llvm] [X86][MC] Compress APX Promoted instrs from evex to legacy encoding to save code size. (PR #77065)

[Phoebe Wang via llvm-commits]

================
@@ -269,28 +299,33 @@ bool EvexToVexInstPass::runOnMachineFunction(MachineFunction &MF) {
            "X86EvexToVex128CompressTable is not sorted!");
     assert(llvm::is_sorted(X86EvexToVex256CompressTable) &&
            "X86EvexToVex256CompressTable is not sorted!");
+    assert(llvm::is_sorted(X86EvexToLegacyCompressTable) &&
+           "X86EvexToLegacyCompressTable is not sorted!");
     TableChecked.store(true, std::memory_order_relaxed);
   }
 #endif
   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
-  if (!ST.hasAVX512())
+  if (!ST.hasAVX512() && !ST.hasEGPR())
     return false;
 
   bool Changed = false;
 
   /// Go over all basic blocks in function and replace
-  /// EVEX encoded instrs by VEX encoding when possible.
+  /// EVEX encoded instrs by VEX/Legacy encoding when possible.
   for (MachineBasicBlock &MBB : MF) {
     // Traverse the basic block.
-    for (MachineInstr &MI : MBB)
+    for (MachineInstr &MI : MBB) {
       Changed |= CompressEvexToVexImpl(MI, ST);
+      Changed |= CompressEVEX2LegacyImpl(MI, ST);
----------------
phoebewang wrote:

This can simplify to 
```
if (CompressEvexToVexImpl(MI, ST) || CompressEVEX2LegacyImpl(MI, ST))
  Changed = true;
```
Because if one instruction can be compressed to VEX, it cannot be compressed to legacy anymore.

https://github.com/llvm/llvm-project/pull/77065