[RISCV][TII] Add and use new hook to simplify/canonicalize instructions after MachineCopyPropagation (#137973)

PR #136875 was posted as a draft PR that handled a subset of these
cases, using the CompressPat mechanism. The consensus from that
discussion (and a conclusion I agree with) is that it would be
beneficial doing this optimisation earlier on, and in a way that isn't
limited just to cases that can be handled by instruction compression.

The most common source for instructions that can be
optimized/canonicalized in this way is through tail duplication in
MachineBlockPlacement followed by machine copy propagation. For RISC-V,
choosing a more canonical instruction allows it to be compressed when it
couldn't be before. There is the potential that it would make other
MI-level optimisations easier.

This modifies ~910 instructions across an llvm-test-suite build
including SPEC2017, targeting rva22u64. Looking at the diff, it seems
there's room for eliminating instructions or further propagating after
this.

Coverage of instructions is based on observations from a script written
to find redundant or improperly canonicalized instructions (though I aim
to support all instructions in a 'group' at once, e.g. MUL* even if I
only saw some variants of MUL in practice).

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index ff75b87..224588b 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -870,6 +870,12 @@ void MachineCopyPropagation::forwardUses(MachineInstr &MI) {
     ++NumCopyForwards;
     Changed = true;
   }
+  // Attempt to canonicalize/optimize the instruction now its arguments have
+  // been mutated.
+  if (TII->simplifyInstruction(MI)) {
+    Changed = true;
+    LLVM_DEBUG(dbgs() << "MCP: After optimizeInstruction: " << MI);
+  }
 }
 
 void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {