[PATCH] D63860: [MachineCSE] Improve CSE on phi node incoming value

[Piotr Sobczak via Phabricator via llvm-commits]

piotr created this revision.
Herald added subscribers: llvm-commits, nhaehnle, jvesely.
Herald added a project: LLVM.

Perform CSE on phi node incoming value, if all of its uses are
phi nodes placed in a BB containing only phi nodes, where the BB's
only successor contains the new use.


Repository:
  rL LLVM

https://reviews.llvm.org/D63860

Files:
  lib/CodeGen/MachineCSE.cpp
  test/CodeGen/AMDGPU/cse-phi-incoming-val.ll


Index: test/CodeGen/AMDGPU/cse-phi-incoming-val.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/cse-phi-incoming-val.ll
@@ -0,0 +1,41 @@
+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=gfx900 -verify-machineinstrs | FileCheck %s
+
+; Check that the redundant immediate MOV instruction
+; (by-product of handling phi nodes) is not found
+; in the generated code due to CSE heuristic.
+
+; CHECK-LABEL: {{^}}mov_opt:
+; CHECK: v_mov_b32_e32 {{v[0-9]+}}, 1.0
+; CHECK: %bb.1:
+; CHECK-NOT: v_mov_b32_e32 {{v[0-9]+}}, 1.0
+; CHECK: BB0_2:
+
+define amdgpu_ps void @mov_opt(i32 %arg, i32 inreg %arg1, i32 inreg %arg2) local_unnamed_addr #0 {
+bb:
+  %tmp = icmp eq i32 %arg1, 0
+  br i1 %tmp, label %bb3, label %bb10
+
+bb3:                                              ; preds = %bb
+  %tmp4 = icmp eq i32 %arg2, 0
+  br i1 %tmp4, label %bb5, label %bb10
+
+bb5:                                              ; preds = %bb3
+  %tmp6 = getelementptr <{ [4294967295 x i32] }>, <{ [4294967295 x i32] }> addrspace(6)* null, i32 0, i32 0, i32 %arg
+  %tmp7 = load i32, i32 addrspace(6)* %tmp6
+  %tmp8 = icmp eq i32 %tmp7, 1
+  br i1 %tmp8, label %bb10, label %bb9
+
+bb9:                                              ; preds = %bb5
+  br label %bb10
+
+bb10:                                             ; preds = %bb9, %bb5, %bb3, %bb
+  %tmp11 = phi float [ 1.000000e+00, %bb3 ], [ 0.000000e+00, %bb9 ], [ 1.000000e+00, %bb ], [ undef, %bb5 ]
+  call void @llvm.amdgcn.exp.f32(i32 immarg 40, i32 immarg 15, float %tmp11, float undef, float undef, float undef, i1 immarg false, i1 immarg false) #0
+  ret void
+}
+
+; Function Attrs: inaccessiblememonly nounwind
+declare void @llvm.amdgcn.exp.f32(i32 immarg, i32 immarg, float, float, float, float, i1 immarg, i1 immarg) #1
+
+attributes #0 = { nounwind }
+attributes #1 = { inaccessiblememonly nounwind }
Index: lib/CodeGen/MachineCSE.cpp
===================================================================
--- lib/CodeGen/MachineCSE.cpp
+++ lib/CodeGen/MachineCSE.cpp
@@ -473,11 +473,33 @@
   }
 
   // Heuristics #3: If the common subexpression is used by PHIs, do not reuse
-  // it unless the defined value is already used in the BB of the new use.
+  // it unless:
+  // a) the defined value is already used in the BB of the new use or
+  // b) all uses of the defined value are phi nodes placed in a BB containing
+  //    only phi nodes, where the BB's only successor contains the new use
+
+  // First find the successor block as defined in point b) above.
+  MachineBasicBlock *BBPhisOnly = nullptr, *BBPhisOnlySucc = nullptr;
+  for (MachineInstr &UseMI : MRI->use_nodbg_instructions(Reg)) {
+    if (!UseMI.isPHI() || (BBPhisOnly && BBPhisOnly != UseMI.getParent())) {
+      BBPhisOnly = nullptr;
+      break;
+    }
+    if (!BBPhisOnly)
+      BBPhisOnly = UseMI.getParent();
+  }
+  if (BBPhisOnly && BBPhisOnly->succ_size() == 1 &&
+      (BBPhisOnly->getFirstNonPHI() == BBPhisOnly->getFirstTerminator()))
+    BBPhisOnlySucc = *BBPhisOnly->succ_begin();
+
   bool HasPHI = false;
-  for (MachineInstr &UseMI : MRI->use_nodbg_instructions(CSReg)) {
-    HasPHI |= UseMI.isPHI();
-    if (UseMI.getParent() == MI->getParent())
+  for (MachineInstr &UseCSMI : MRI->use_nodbg_instructions(CSReg)) {
+    HasPHI |= UseCSMI.isPHI();
+    // a) the defined value is already used in the BB of the new use
+    if (UseCSMI.getParent() == MI->getParent())
+      return true;
+    // b) the BB's only successor contains the new use
+    if (UseCSMI.getParent() == BBPhisOnlySucc)
       return true;
   }
 


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