Merge memtag instructions with adjacent stack slots.

Summary:
Detect a run of memory tagging instructions for adjacent stack frame slots,
and replace them with a shorter instruction sequence
* replace STG + STG with ST2G
* replace STGloop + STGloop with STGloop

This code needs to run when stack slot offsets are already known, but before
FrameIndex operands in STG instructions are eliminated; that's the
reason for the new hook in PrologueEpilogue.

This change modifies STGloop and STZGloop pseudos to take the size as an
immediate integer operand, and adds _untied variants of those pseudos
that are allowed to take the base address as a FI operand. This is needed to
simplify recognizing an STGloop instruction as operating on a stack slot
post-regalloc.

This improves memtag code size by ~0.25%, and it looks like an additional ~0.1%
is possible by rearranging the stack frame such that consecutive STG
instructions reference adjacent slots (patch pending).

Reviewers: pcc, ostannard

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70286

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/PrologEpilogInserter.cpp b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
index 3909b57..d583643 100644
--- a/llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -259,6 +259,10 @@ bool PEI::runOnMachineFunction(MachineFunction &MF) {
   for (auto &I : EntryDbgValues)
     I.first->insert(I.first->begin(), I.second.begin(), I.second.end());
 
+  // Allow the target machine to make final modifications to the function
+  // before the frame layout is finalized.
+  TFI->processFunctionBeforeFrameIndicesReplaced(MF, RS);
+
   // Replace all MO_FrameIndex operands with physical register references
   // and actual offsets.
   //