[PATCH] D81875: [GlobalISel] Look through extends etc in CombinerHelper::matchConstantOp

[Jessica Paquette via Phabricator via llvm-commits]

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

It's possible to end up with a zext or something in the way of a G_CONSTANT, even pre-legalization. This can happen with memsets.

e.g.

https://godbolt.org/z/Bjc8cw

To make sure we can catch these cases, use `getConstantVRegValWithLookThrough` instead of `mi_match`.


https://reviews.llvm.org/D81875

Files:
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir
+++ llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir
@@ -286,4 +286,24 @@
     %op:_(s32) = G_SUB %x(s32), %cst
     $w0 = COPY %op(s32)
     RET_ReallyLR implicit $w0
+
+...
+---
+name:            look_through_zext
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0
+    ; CHECK-LABEL: name: look_through_zext
+    ; CHECK: liveins: $x0
+    ; CHECK: %zero:_(s8) = G_CONSTANT i8 0
+    ; CHECK: %zext_zero:_(s64) = G_ZEXT %zero(s8)
+    ; CHECK: $x0 = COPY %zext_zero(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %zero:_(s8) = G_CONSTANT i8 0
+    %zext_zero:_(s64) = G_ZEXT %zero(s8)
+    %c:_(s64) = G_CONSTANT i64 72340172838076673
+    %mul:_(s64) = G_MUL %c, %zext_zero
+    $x0 = COPY %mul(s64)
+    RET_ReallyLR implicit $x0
 ...
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1612,8 +1612,9 @@
 bool CombinerHelper::matchConstantOp(const MachineOperand &MOP, int64_t C) {
   if (!MOP.isReg())
     return false;
-  int64_t Cst;
-  return mi_match(MOP.getReg(), MRI, m_ICst(Cst)) && Cst == C;
+  // MIPatternMatch doesn't let us look through G_ZEXT etc.
+  auto ValAndVReg = getConstantVRegValWithLookThrough(MOP.getReg(), MRI);
+  return ValAndVReg && ValAndVReg->Value == C;
 }
 
 bool CombinerHelper::replaceSingleDefInstWithOperand(MachineInstr &MI,


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D81875.270843.patch
Type: text/x-patch
Size: 1728 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200615/f4dc1857/attachment.bin>