[PATCH] D65360: [AArch64][GlobalISel] Eliminate redundant G_ZEXT when the source is implicitly zext-loaded

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.
aemerson added a reviewer: paquette.
Herald added subscribers: Petar.Avramovic, hiraditya, kristof.beyls, javed.absar, rovka.
Herald added a project: LLVM.

These cases can come up when the extending loads combiner doesn't combine a zext(load) to a zextload op, due to some other operation being in between, which then gets simplified at a later stage.

This gives code size improvements on -O0 CTMark, geomean 0.1%.
sqlite3: -0.4%
consumer-typeset: -0.3%
clamscan: -0.2%
lencod: -0.1%


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D65360

Files:
  llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/select-redundant-zext-of-load.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/select-redundant-zext-of-load.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/select-redundant-zext-of-load.mir
@@ -0,0 +1,48 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64-- -O0 -run-pass=instruction-select -verify-machineinstrs %s -global-isel-abort=1 -o - | FileCheck %s
+---
+name:            redundant_zext_8
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $x0
+
+    ; CHECK-LABEL: name: redundant_zext_8
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:gpr64sp = COPY $x0
+    ; CHECK: [[LDRBBui:%[0-9]+]]:gpr32 = LDRBBui [[COPY]], 0 :: (load 1)
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr32all = COPY [[LDRBBui]]
+    ; CHECK: $w0 = COPY [[COPY1]]
+    ; CHECK: RET_ReallyLR implicit $w0
+    %1:gpr(p0) = COPY $x0
+    %2:gpr(s8) = G_LOAD %1(p0) :: (load 1)
+    %3:gpr(s32) = G_ZEXT %2(s8)
+    $w0 = COPY %3(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            redundant_zext_16
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $x0
+
+    ; CHECK-LABEL: name: redundant_zext_16
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:gpr64sp = COPY $x0
+    ; CHECK: [[LDRHHui:%[0-9]+]]:gpr32 = LDRHHui [[COPY]], 0 :: (load 2)
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr32all = COPY [[LDRHHui]]
+    ; CHECK: $w0 = COPY [[COPY1]]
+    ; CHECK: RET_ReallyLR implicit $w0
+    %1:gpr(p0) = COPY $x0
+    %2:gpr(s16) = G_LOAD %1(p0) :: (load 2)
+    %3:gpr(s32) = G_ZEXT %2(s16)
+    $w0 = COPY %3(s32)
+    RET_ReallyLR implicit $w0
+
+...
Index: llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -2044,6 +2044,20 @@
     if (DstTy.isVector())
       return false; // Should be handled by imported patterns.
 
+    // First check if we're extending the result of a load which has a dest type
+    // smaller than 32 bits, then this zext is redundant. GPR32 is the smallest
+    // GPR register on AArch64 and all loads which are smaller automatically
+    // zero-extend the upper bits. E.g.
+    // %v(s8) = G_LOAD %p, :: (load 1)
+    // %v2(s32) = G_ZEXT %v(s8)
+    auto *LoadMI = getOpcodeDef(TargetOpcode::G_LOAD, SrcReg, MRI);
+    if (LoadMI) {
+      const MachineMemOperand *MemOp = *LoadMI->memoperands_begin();
+      unsigned BytesLoaded = MemOp->getSize();
+      if (BytesLoaded < 4 && SrcTy.getSizeInBytes() == BytesLoaded)
+        return selectCopy(I, TII, MRI, TRI, RBI);
+    }
+
     if (DstSize == 64) {
       // FIXME: Can we avoid manually doing this?
       if (!RBI.constrainGenericRegister(SrcReg, AArch64::GPR32RegClass, MRI)) {


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D65360.212032.patch
Type: text/x-patch
Size: 2989 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190727/4a02ca06/attachment.bin>