[PATCH] D95565: [GlobalISel] Add hint elimination combine

[Jessica Paquette via Phabricator via llvm-commits]

paquette created this revision.
paquette added reviewers: aemerson, arsenm.
Herald added subscribers: hiraditya, rovka.
paquette requested review of this revision.
Herald added a subscriber: wdng.
Herald added a project: LLVM.

This adds a hint elimination combine, which changes

  %x = ...
  %y = G_ASSERT_ZEXT %x, n
  ...
  %z = G_FOO ... %y ...

into

  %x = ...
  ...
  %z = G_FOO ... %x

This is intended to run in a late combiner, post-legalization.


https://reviews.llvm.org/D95565

Files:
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/Target/AArch64/AArch64Combine.td
  llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-lowering-hint.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-lowering-hint.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-lowering-hint.mir
@@ -0,0 +1,22 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64 -run-pass=aarch64-postlegalizer-lowering -verify-machineinstrs %s -o - | FileCheck %s
+#
+# Check that we remove hint instructions.
+
+---
+name:            assert_zext
+legalized:       true
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $w0, $w1
+    ; CHECK-LABEL: name: assert_zext
+    ; CHECK: liveins: $w0, $w1
+    ; CHECK: %copy:_(s32) = COPY $w1
+    ; CHECK: $w0 = COPY %copy(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %copy:_(s32) = COPY $w1
+    %hint:_(s32) = G_ASSERT_ZEXT %copy, 16
+    $w0 = COPY %hint
+    RET_ReallyLR implicit $w0
+...
Index: llvm/lib/Target/AArch64/AArch64Combine.td
===================================================================
--- llvm/lib/Target/AArch64/AArch64Combine.td
+++ llvm/lib/Target/AArch64/AArch64Combine.td
@@ -125,7 +125,7 @@
 def AArch64PostLegalizerLoweringHelper
     : GICombinerHelper<"AArch64GenPostLegalizerLoweringHelper",
                        [shuffle_vector_pseudos, vashr_vlshr_imm,
-                        icmp_lowering, form_duplane]> {
+                        icmp_lowering, form_duplane, eliminate_hints]> {
   let DisableRuleOption = "aarch64postlegalizerlowering-disable-rule";
 }
 
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -557,6 +557,21 @@
 def insert_vec_elt_combines : GICombineGroup<
                             [combine_insert_vec_elts_build_vector]>;
 
+// Remove hint instructions in the same way as copies.
+//
+// %x = ...
+// %y = G_ASSERT_ZEXT %x, n
+// ... = G_FOO %y, ...
+// ->
+//
+// %x = ...
+// ... G_FOO %x, ...
+def eliminate_hints: GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_ASSERT_ZEXT):$root,
+         [{ return true; }]),
+  (apply [{ Helper.applyCombineCopy(*${root}); }])>;
+
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,


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