[AMDGPU] Increase detection range for s_mov, v_cmpx transformation.

[tsymalla]

Summary

We found that it might be beneficial to have the SIOptimizeExecMasking
pass detect more cases where v_cmp, s_and_saveexec patterns can be
transformed to s_mov, v_cmpx patterns. Currently, the search range
for finding a fitting v_cmp instruction is 5, however, this is doubled
to 10 here.

[foad]

Seems obviously fine, since we have seen real world cases where it helps.

[tsymalla]

Put the limit up to 20 instructions.

[foad]

llvm/lib/Target/AMDGPU/SIOptimizeExecMasking.cpp

322

Pre-existing problem: debug instructions (A->isDebugInstr()) should not count towards the search limit, because we need to get identical codegen with and without debug instructions present.

[tsymalla]

Added handling for debug instrs.

[foad]

LGTM, thanks.

llvm/lib/Target/AMDGPU/SIOptimizeExecMasking.cpp

 // the beginning of the BB is reached or Pred evaluates to true - which can be
 // an arbitrary condition based on the current MachineInstr, for instance an
 // target instruction. Breaks prematurely by returning nullptr if  one of the
 // registers given in NonModifiableRegs is modified by the current instruction.
 static MachineInstr *
 findInstrBackwards(MachineInstr &Origin,
                    std::function<bool(MachineInstr *)> Pred,
                    ArrayRef<MCRegister> NonModifiableRegs,
-                   const SIRegisterInfo *TRI, unsigned MaxInstructions = 5) {
+                   const SIRegisterInfo *TRI, unsigned MaxInstructions = 20) {
   MachineBasicBlock::reverse_iterator A = Origin.getReverseIterator(),
                                       E = Origin.getParent()->rend();
   unsigned CurrentIteration = 0;
 
   for (++A; CurrentIteration < MaxInstructions && A != E; ++A) {
+    if (A->isDebugInstr())
+      continue;
+    
     if (Pred(&*A))
       return &*A;
 
     for (MCRegister Reg : NonModifiableRegs) {
       if (A->modifiesRegister(Reg, TRI))
         return nullptr;
     }
     
     ++CurrentIteration;
   }
 
   return nullptr;
 }
 
 
 // Determine if a register Reg is not re-defined and still in use
 // in the range (Stop..Start].

llvm/test/CodeGen/AMDGPU/vcmp-saveexec-to-vcmpx.mir

 # RUN: llc -march=amdgcn -mcpu=gfx1010 -mattr=-wavefrontsize32,+wavefrontsize64 -run-pass=si-optimize-exec-masking -verify-machineinstrs %s -o - | FileCheck --check-prefixes=GCN,GFX1010 %s
 # RUN: llc -march=amdgcn -mcpu=gfx1030 -mattr=-wavefrontsize32,+wavefrontsize64 -run-pass=si-optimize-exec-masking -verify-machineinstrs %s -o - | FileCheck --check-prefixes=GCN,GFX1030 %s
-
 ---
 
 # After the Optimize exec masking (post-RA) pass, there's a change of having v_cmpx instructions
 # being introduced whenever there's a sequence of v_cmp and s_and_saveexec instructions
 # which can be safely replaced in various cases.
 # However, it is not safe to do so when the generated code sequence would omit part of the EXEC mask
 # which could occur when a subset of EXEC is used as input operand in the v_cmp instruction.
 # The idea behind this test is to check if the subregisters are correctly handled here.

 tracksRegLiveness: true
 body: |
   bb.0:
     liveins: $vgpr0
     renamable $sgpr0_sgpr1 = V_CMP_LT_F32_e64 0, 953267991, 2, $vgpr0, 0, implicit $mode, implicit $exec
     $sgpr2_sgpr3 = COPY $exec, implicit-def $exec
     $sgpr2_sgpr3 = S_AND_B64 killed renamable $sgpr2_sgpr3, killed renamable $sgpr0_sgpr1, implicit-def dead $scc
     $exec = S_MOV_B64_term killed renamable $sgpr2_sgpr3
+...
+
+---
+
+# Check if the sequence will be optimized even with more than 5 (unrelated) instructions inbetween the v_cmp and s_and_saveexec.
+
+# GCN-LABEL: name: vcmp_saveexec_to_mov_vcmpx_check_many_instrs
+# GFX1010: V_CMP_LT_F32_e64
+# GFX1010: S_AND_SAVEEXEC_B64
+# GFX1030: S_MOV_B64
+# GFX1030: V_CMPX_LT_F32_nosdst_e64 0, 953267991, 2
+name: vcmp_saveexec_to_mov_vcmpx_check_many_instrs
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $vgpr0, $sgpr2, $vgpr1
+    renamable $sgpr0_sgpr1 = V_CMP_LT_F32_e64 0, 953267991, 2, $vgpr0, 0, implicit $mode, implicit $exec
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $vgpr1 = V_WRITELANE_B32 0, $sgpr2, $vgpr1
+    $sgpr2_sgpr3 = COPY $exec, implicit-def $exec
+    $sgpr2_sgpr3 = S_AND_B64 killed renamable $sgpr2_sgpr3, killed renamable $sgpr0_sgpr1, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed renamable $sgpr2_sgpr3