[AMDGPU] Allow selection of BITOP3 for some 2 opcodes and B32 cases (llvm#122267)

This came up in downstream static analysis - as a dead code.

Admittedly, it depends on what the intention was when checking for [`if
(NumOpcodes == 2 &&
IsB32)`](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp#L3792C3-L3792C32)
and I took a guess that for certain cases the selection should take
place.

If that's incorrect, that whole if statement can be removed, as it is
after a check for: [`if (NumOpcodes <
4)`](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp#L3788)

Author: Jakub Chlanda

llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp

@@ -3782,13 +3782,7 @@ bool AMDGPUInstructionSelector::selectBITOP3(MachineInstr &MI) const {
   if (NumOpcodes < 2 || Src.empty())
     return false;
 
-  // For a uniform case threshold should be higher to account for moves between
-  // VGPRs and SGPRs. It needs one operand in a VGPR, rest two can be in SGPRs
-  // and a readtfirstlane after.
-  if (NumOpcodes < 4)
-    return false;
-
-  bool IsB32 = MRI->getType(DstReg) == LLT::scalar(32);
+  const bool IsB32 = MRI->getType(DstReg) == LLT::scalar(32);
   if (NumOpcodes == 2 && IsB32) {
     // Avoid using BITOP3 for OR3, XOR3, AND_OR. This is not faster but makes
     // asm more readable. This cannot be modeled with AddedComplexity because
@@ -3797,6 +3791,11 @@ bool AMDGPUInstructionSelector::selectBITOP3(MachineInstr &MI) const {
         mi_match(MI, *MRI, m_GOr(m_GOr(m_Reg(), m_Reg()), m_Reg())) ||
         mi_match(MI, *MRI, m_GOr(m_GAnd(m_Reg(), m_Reg()), m_Reg())))
       return false;
+  } else if (NumOpcodes < 4) {
+    // For a uniform case threshold should be higher to account for moves
+    // between VGPRs and SGPRs. It needs one operand in a VGPR, rest two can be
+    // in SGPRs and a readtfirstlane after.
+    return false;
   }
 
   unsigned Opc = IsB32 ? AMDGPU::V_BITOP3_B32_e64 : AMDGPU::V_BITOP3_B16_e64;

llvm/test/CodeGen/AMDGPU/bitop3.ll

@@ -52,8 +52,7 @@ define amdgpu_ps float @not_and_and_and(i32 %a, i32 %b, i32 %c) {
 ;
 ; GFX950-GISEL-LABEL: not_and_and_and:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v0, v0
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v2
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v2, v0 bitop3:0xc
 ; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v1
 ; GFX950-GISEL-NEXT:    ; return to shader part epilog
   %nota = xor i32 %a, -1
@@ -103,8 +102,7 @@ define amdgpu_ps float @and_and_not_and(i32 %a, i32 %b, i32 %c) {
 ;
 ; GFX950-GISEL-LABEL: and_and_not_and:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v2, v2
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v2
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v2, v0 bitop3:0x30
 ; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v1
 ; GFX950-GISEL-NEXT:    ; return to shader part epilog
   %notc = xor i32 %c, -1
@@ -122,8 +120,7 @@ define amdgpu_ps float @and_and_and(i32 %a, i32 %b, i32 %c) {
 ;
 ; GFX950-GISEL-LABEL: and_and_and:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v2
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v1
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v1, v2 bitop3:0x80
 ; GFX950-GISEL-NEXT:    ; return to shader part epilog
   %and1 = and i32 %a, %c
   %and2 = and i32 %and1, %b
@@ -141,8 +138,7 @@ define amdgpu_ps float @test_12(i32 %a, i32 %b) {
 ;
 ; GFX950-GISEL-LABEL: test_12:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v0, v0
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v1
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v1, v0 bitop3:0xc
 ; GFX950-GISEL-NEXT:    ; return to shader part epilog
   %nota = xor i32 %a, -1
   %and1 = and i32 %nota, %b
@@ -214,9 +210,11 @@ define amdgpu_ps float @test_12_src_overflow(i32 %a, i32 %b, i32 %c) {
 ;
 ; GFX950-GISEL-LABEL: test_12_src_overflow:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v0, v0
-; GFX950-GISEL-NEXT:    v_bfi_b32 v0, v2, v0, v0
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v0, v1
+; GFX950-GISEL-NEXT:    v_not_b32_e32 v3, v0
+; GFX950-GISEL-NEXT:    v_not_b32_e32 v4, v2
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v2, v0 bitop3:0xc
+; GFX950-GISEL-NEXT:    v_and_b32_e32 v2, v3, v4
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v0, v0, v1, v2 bitop3:0xc8
 ; GFX950-GISEL-NEXT:    ; return to shader part epilog
   %nota = xor i32 %a, -1
   %notc = xor i32 %c, -1
@@ -242,11 +240,9 @@ define amdgpu_ps float @test_100_src_overflow(i32 %a, i32 %b, i32 %c) {
 ;
 ; GFX950-GISEL-LABEL: test_100_src_overflow:
 ; GFX950-GISEL:       ; %bb.0:
-; GFX950-GISEL-NEXT:    v_or_b32_e32 v3, v2, v0
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v3, v3
-; GFX950-GISEL-NEXT:    v_not_b32_e32 v4, v1
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v3, v2, v0, v2 bitop3:3
 ; GFX950-GISEL-NEXT:    v_and_b32_e32 v3, v1, v3
-; GFX950-GISEL-NEXT:    v_and_b32_e32 v4, v0, v4
+; GFX950-GISEL-NEXT:    v_bitop3_b32 v4, v0, v1, v0 bitop3:0x30
 ; GFX950-GISEL-NEXT:    v_and_b32_e32 v0, v1, v0
 ; GFX950-GISEL-NEXT:    v_not_b32_e32 v1, v2
 ; GFX950-GISEL-NEXT:    v_and_b32_e32 v4, v4, v2