[PATCH] D96031: [GlobalISel] Combine zext(trunc x) to x

[Petar Avramovic via Phabricator via llvm-commits]

Petar.Avramovic created this revision.
Petar.Avramovic added reviewers: foad, arsen, aemerson, paquette.
Herald added subscribers: kerbowa, hiraditya, rovka, nhaehnle, jvesely.
Petar.Avramovic requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Combine zext(trunc x) to x when truncated bits are known to be zero.


https://reviews.llvm.org/D96031

Files:
  llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/test/CodeGen/AMDGPU/GlobalISel/shl-ext-reduce.ll


Index: llvm/test/CodeGen/AMDGPU/GlobalISel/shl-ext-reduce.ll
===================================================================
--- llvm/test/CodeGen/AMDGPU/GlobalISel/shl-ext-reduce.ll
+++ llvm/test/CodeGen/AMDGPU/GlobalISel/shl-ext-reduce.ll
@@ -404,18 +404,14 @@
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_and_b32 s0, s0, 0xffff
 ; GFX8-NEXT:    s_and_b32 s0, s0, 0x3fff
-; GFX8-NEXT:    s_bfe_u32 s1, 2, 0x100000
-; GFX8-NEXT:    s_lshl_b32 s0, s0, s1
-; GFX8-NEXT:    s_bfe_u32 s0, s0, 0x100000
+; GFX8-NEXT:    s_lshl_b32 s0, s0, 2
 ; GFX8-NEXT:    ; return to shader part epilog
 ;
 ; GFX9-LABEL: s_shl_i32_zext_i16:
 ; GFX9:       ; %bb.0:
 ; GFX9-NEXT:    s_and_b32 s0, s0, 0xffff
 ; GFX9-NEXT:    s_and_b32 s0, s0, 0x3fff
-; GFX9-NEXT:    s_bfe_u32 s1, 2, 0x100000
-; GFX9-NEXT:    s_lshl_b32 s0, s0, s1
-; GFX9-NEXT:    s_bfe_u32 s0, s0, 0x100000
+; GFX9-NEXT:    s_lshl_b32 s0, s0, 2
 ; GFX9-NEXT:    ; return to shader part epilog
   %and = and i16 %x, 16383
   %ext = zext i16 %and to i32
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -2301,6 +2301,19 @@
   return true;
 }
 
+bool CombinerHelper::matchCombineZextTrunc(MachineInstr &MI, Register &Reg) {
+  assert(MI.getOpcode() == TargetOpcode::G_ZEXT && "Expected a G_ZEXT");
+  Register DstReg = MI.getOperand(0).getReg();
+  Register SrcReg = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(DstReg);
+  if (mi_match(SrcReg, MRI,
+               m_GTrunc(m_all_of(m_Reg(Reg), m_SpecificType(DstTy))))) {
+    return KB->getKnownBits(Reg).countMinLeadingZeros() >=
+           DstTy.getSizeInBits() - MRI.getType(SrcReg).getSizeInBits();
+  }
+  return false;
+}
+
 bool CombinerHelper::matchCombineExtOfExt(
     MachineInstr &MI, std::tuple<Register, unsigned> &MatchInfo) {
   assert((MI.getOpcode() == TargetOpcode::G_ANYEXT ||
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -426,6 +426,16 @@
   (apply [{ return Helper.applyCombineAnyExtTrunc(*${root}, ${matchinfo}); }])
 >;
 
+// Fold (zext (trunc x)) -> x if the source type is same as the destination type
+// and truncated bits are known to be zero.
+def zext_trunc_fold_matchinfo : GIDefMatchData<"Register">;
+def zext_trunc_fold: GICombineRule <
+  (defs root:$root, zext_trunc_fold_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_ZEXT):$root,
+         [{ return Helper.matchCombineZextTrunc(*${root}, ${matchinfo}); }]),
+  (apply [{ return Helper.replaceSingleDefInstWithReg(*${root}, ${matchinfo}); }])
+>;
+
 // Fold ([asz]ext ([asz]ext x)) -> ([asz]ext x).
 def ext_ext_fold_matchinfo : GIDefMatchData<"std::tuple<Register, unsigned>">;
 def ext_ext_fold: GICombineRule <
@@ -575,7 +585,8 @@
 def const_combines : GICombineGroup<[constant_fp_op, const_ptradd_to_i2p]>;
 
 def known_bits_simplifications : GICombineGroup<[
-  redundant_and, redundant_sext_inreg, redundant_or, urem_pow2_to_mask]>;
+  redundant_and, redundant_sext_inreg, redundant_or, urem_pow2_to_mask,
+  zext_trunc_fold]>;
 
 def width_reduction_combines : GICombineGroup<[reduce_shl_of_extend]>;
 
Index: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -316,6 +316,9 @@
   bool matchCombineAnyExtTrunc(MachineInstr &MI, Register &Reg);
   bool applyCombineAnyExtTrunc(MachineInstr &MI, Register &Reg);
 
+  /// Transform zext(trunc(x)) to x.
+  bool matchCombineZextTrunc(MachineInstr &MI, Register &Reg);
+
   /// Transform [asz]ext([asz]ext(x)) to [asz]ext x.
   bool matchCombineExtOfExt(MachineInstr &MI,
                             std::tuple<Register, unsigned> &MatchInfo);


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