[MIPS GlobalISel] Select count trailing zeros

[Petar.Avramovic]

Summary

llvm.cttz.<type> intrinsic has additional i1 argument is_zero_undef,
it tells whether zero as the first argument produces a defined result.
G_CTTZ is generated from llvm.cttz.<type> (<type> <src>, i1 false)
intrinsics, clang generates these intrinsics from builtin_ctz and
builtin_ctzll.
G_CTTZ_ZERO_UNDEF comes from llvm.cttz.<type> (<type> <src>, i1 true).
Clang generates such intrinsics as parts of expansion of builtin_ffs
and builtin_ffsll. It is also traditionally part of and many
algorithms that are now predicated on avoiding zero-value inputs.

Add narrow scalar (algorithm uses G_CTTZ_ZERO_UNDEF) for G_CTTZ.
Lower G_CTTZ and G_CTTZ_ZERO_UNDEF for MIPS32.

[Petar.Avramovic]

Add check that OrigType is scalar and NarrowTy == OrigType/2. Fix build unmerge.

[Petar.Avramovic]

Add comment in narrow scalar for cttz.

[atanasyan]

LGTM

llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h

public:
   LegalizeResult narrowScalarMul(MachineInstr &MI, LLT Ty);
   LegalizeResult narrowScalarExtract(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult narrowScalarInsert(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
 
   LegalizeResult narrowScalarBasic(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult narrowScalarExt(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult narrowScalarSelect(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult narrowScalarCTLZ(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
+  LegalizeResult narrowScalarCTTZ(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
 
   LegalizeResult lowerBitcast(MachineInstr &MI);
   LegalizeResult lowerBitCount(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
 
   LegalizeResult lowerU64ToF32BitOps(MachineInstr &MI);
   LegalizeResult lowerUITOFP(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult lowerSITOFP(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult lowerFPTOUI(MachineInstr &MI, unsigned TypeIdx, LLT Ty);

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
   case TargetOpcode::G_CTLZ_ZERO_UNDEF:
   case TargetOpcode::G_CTTZ:
   case TargetOpcode::G_CTTZ_ZERO_UNDEF:
   case TargetOpcode::G_CTPOP:
     if (TypeIdx == 1)
       switch (MI.getOpcode()) {
       case TargetOpcode::G_CTLZ:
         return narrowScalarCTLZ(MI, TypeIdx, NarrowTy);
+      case TargetOpcode::G_CTTZ:
+        return narrowScalarCTTZ(MI, TypeIdx, NarrowTy);
       default:
         return UnableToLegalize;
       }
 
     Observer.changingInstr(MI);
     narrowScalarDst(MI, NarrowTy, 0, TargetOpcode::G_ZEXT);
     Observer.changedInstr(MI);
     return Legalized;
if (SrcTy.isScalar() && SrcTy.getSizeInBits() == 2 * NarrowSize) {
     MI.eraseFromParent();
     return Legalized;
   }
 
   return UnableToLegalize;
 }
 
 LegalizerHelper::LegalizeResult
+LegalizerHelper::narrowScalarCTTZ(MachineInstr &MI, unsigned TypeIdx,
+                                  LLT NarrowTy) {
+  if (TypeIdx != 1)
+    return UnableToLegalize;
+
+  LLT SrcTy = MRI.getType(MI.getOperand(1).getReg());
+  unsigned NarrowSize = NarrowTy.getSizeInBits();
+
+  if (SrcTy.isScalar() && SrcTy.getSizeInBits() == 2 * NarrowSize) {
+    MachineIRBuilder &B = MIRBuilder;
+    auto UnmergeSrc = B.buildUnmerge(NarrowTy, MI.getOperand(1));
+    // cttz(Hi:Lo) -> Lo == 0 ? (cttz(Hi) + NarrowSize) : cttz(Lo)
+    auto C_0 = B.buildConstant(NarrowTy, 0);
+    auto LoIsZero = B.buildICmp(CmpInst::ICMP_EQ, LLT::scalar(1),
+                                UnmergeSrc.getReg(0), C_0);
+    auto HiCTTZ = B.buildCTTZ(NarrowTy, UnmergeSrc.getReg(1));
+    auto C_NarrowSize = B.buildConstant(NarrowTy, NarrowSize);
+    auto LoIsZeroCTTZ = B.buildAdd(NarrowTy, HiCTTZ, C_NarrowSize);
+    auto LoCTTZ = B.buildCTTZ_ZERO_UNDEF(NarrowTy, UnmergeSrc.getReg(0));
+    auto LoOut = B.buildSelect(NarrowTy, LoIsZero, LoIsZeroCTTZ, LoCTTZ);
+
+    B.buildMerge(MI.getOperand(0), {LoOut.getReg(0), C_0.getReg(0)});
+
+    MI.eraseFromParent();
+    return Legalized;
+  }
+
+  return UnableToLegalize;
+}
+
+LegalizerHelper::LegalizeResult
 LegalizerHelper::lowerBitCount(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
   unsigned Opc = MI.getOpcode();
   auto &TII = *MI.getMF()->getSubtarget().getInstrInfo();
   auto isSupported = [this](const LegalityQuery &Q) {
     auto QAction = LI.getAction(Q).Action;
     return QAction == Legal || QAction == Libcall || QAction == Custom;
   };
   switch (Opc) {

llvm/lib/Target/Mips/MipsLegalizerInfo.cpp

getActionDefinitionsBuilder(G_BITREVERSE)
       .maxScalar(0, s32);
 
   getActionDefinitionsBuilder(G_CTLZ)
       .legalFor({{s32, s32}})
       .maxScalar(1, s32);
   getActionDefinitionsBuilder(G_CTLZ_ZERO_UNDEF)
       .lowerFor({{s32, s32}});
 
+  getActionDefinitionsBuilder(G_CTTZ)
+      .lowerFor({{s32, s32}})
+      .maxScalar(1, s32);
+  getActionDefinitionsBuilder(G_CTTZ_ZERO_UNDEF)
+      .lowerFor({{s32, s32}, {s64, s64}});
+
   // FP instructions
   getActionDefinitionsBuilder(G_FCONSTANT)
       .legalFor({s32, s64});
 
   getActionDefinitionsBuilder({G_FADD, G_FSUB, G_FMUL, G_FDIV, G_FABS, G_FSQRT})
       .legalIf([=, &ST](const LegalityQuery &Query) {
         if (CheckTyN(0, Query, {s32, s64}))
           return true;

llvm/test/CodeGen/Mips/GlobalISel/legalizer/cttz.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -O0 -mtriple=mipsel-linux-gnu -run-pass=legalizer -verify-machineinstrs %s -o - | FileCheck %s -check-prefixes=MIPS32
+---
+name:            cttz_i32
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $a0
+
+    ; MIPS32-LABEL: name: cttz_i32
+    ; MIPS32: liveins: $a0
+    ; MIPS32: [[COPY:%[0-9]+]]:_(s32) = COPY $a0
+    ; MIPS32: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 -1
+    ; MIPS32: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY]], [[C]]
+    ; MIPS32: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[C]]
+    ; MIPS32: [[AND:%[0-9]+]]:_(s32) = G_AND [[XOR]], [[ADD]]
+    ; MIPS32: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; MIPS32: [[CTLZ:%[0-9]+]]:_(s32) = G_CTLZ [[AND]](s32)
+    ; MIPS32: [[SUB:%[0-9]+]]:_(s32) = G_SUB [[C1]], [[CTLZ]]
+    ; MIPS32: $v0 = COPY [[SUB]](s32)
+    ; MIPS32: RetRA implicit $v0
+    %0:_(s32) = COPY $a0
+    %1:_(s32) = G_CTTZ %0(s32)
+    $v0 = COPY %1(s32)
+    RetRA implicit $v0
+
+...
+---
+name:            cttz_i64
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $a0, $a1
+
+    ; MIPS32-LABEL: name: cttz_i64
+    ; MIPS32: liveins: $a0, $a1
+    ; MIPS32: [[COPY:%[0-9]+]]:_(s32) = COPY $a0
+    ; MIPS32: [[COPY1:%[0-9]+]]:_(s32) = COPY $a1
+    ; MIPS32: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; MIPS32: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(eq), [[COPY]](s32), [[C]]
+    ; MIPS32: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 -1
+    ; MIPS32: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY1]], [[C1]]
+    ; MIPS32: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY1]], [[C1]]
+    ; MIPS32: [[AND:%[0-9]+]]:_(s32) = G_AND [[XOR]], [[ADD]]
+    ; MIPS32: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; MIPS32: [[CTLZ:%[0-9]+]]:_(s32) = G_CTLZ [[AND]](s32)
+    ; MIPS32: [[SUB:%[0-9]+]]:_(s32) = G_SUB [[C2]], [[CTLZ]]
+    ; MIPS32: [[ADD1:%[0-9]+]]:_(s32) = G_ADD [[SUB]], [[C2]]
+    ; MIPS32: [[XOR1:%[0-9]+]]:_(s32) = G_XOR [[COPY]], [[C1]]
+    ; MIPS32: [[ADD2:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[C1]]
+    ; MIPS32: [[AND1:%[0-9]+]]:_(s32) = G_AND [[XOR1]], [[ADD2]]
+    ; MIPS32: [[CTLZ1:%[0-9]+]]:_(s32) = G_CTLZ [[AND1]](s32)
+    ; MIPS32: [[SUB1:%[0-9]+]]:_(s32) = G_SUB [[C2]], [[CTLZ1]]
+    ; MIPS32: [[C3:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+    ; MIPS32: [[COPY2:%[0-9]+]]:_(s32) = COPY [[ICMP]](s32)
+    ; MIPS32: [[AND2:%[0-9]+]]:_(s32) = G_AND [[COPY2]], [[C3]]
+    ; MIPS32: [[SELECT:%[0-9]+]]:_(s32) = G_SELECT [[AND2]](s32), [[ADD1]], [[SUB1]]
+    ; MIPS32: $v0 = COPY [[SELECT]](s32)
+    ; MIPS32: $v1 = COPY [[C]](s32)
+    ; MIPS32: RetRA implicit $v0, implicit $v1
+    %1:_(s32) = COPY $a0
+    %2:_(s32) = COPY $a1
+    %0:_(s64) = G_MERGE_VALUES %1(s32), %2(s32)
+    %3:_(s64) = G_CTTZ %0(s64)
+    %4:_(s32), %5:_(s32) = G_UNMERGE_VALUES %3(s64)
+    $v0 = COPY %4(s32)
+    $v1 = COPY %5(s32)
+    RetRA implicit $v0, implicit $v1
+
+...
+---
+name:            ffs_i32_expansion
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $a0
+
+    ; MIPS32-LABEL: name: ffs_i32_expansion
+    ; MIPS32: liveins: $a0
+    ; MIPS32: [[COPY:%[0-9]+]]:_(s32) = COPY $a0
+    ; MIPS32: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+    ; MIPS32: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; MIPS32: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 -1
+    ; MIPS32: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY]], [[C2]]
+    ; MIPS32: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[C2]]
+    ; MIPS32: [[AND:%[0-9]+]]:_(s32) = G_AND [[XOR]], [[ADD]]
+    ; MIPS32: [[C3:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; MIPS32: [[CTLZ:%[0-9]+]]:_(s32) = G_CTLZ [[AND]](s32)
+    ; MIPS32: [[SUB:%[0-9]+]]:_(s32) = G_SUB [[C3]], [[CTLZ]]
+    ; MIPS32: [[ADD1:%[0-9]+]]:_(s32) = nuw nsw G_ADD [[SUB]], [[C]]
+    ; MIPS32: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(eq), [[COPY]](s32), [[C1]]
+    ; MIPS32: [[COPY1:%[0-9]+]]:_(s32) = COPY [[ICMP]](s32)
+    ; MIPS32: [[AND1:%[0-9]+]]:_(s32) = G_AND [[COPY1]], [[C]]
+    ; MIPS32: [[SELECT:%[0-9]+]]:_(s32) = G_SELECT [[AND1]](s32), [[C1]], [[ADD1]]
+    ; MIPS32: $v0 = COPY [[SELECT]](s32)
+    ; MIPS32: RetRA implicit $v0
+    %0:_(s32) = COPY $a0
+    %2:_(s32) = G_CONSTANT i32 1
+    %4:_(s32) = G_CONSTANT i32 0
+    %1:_(s32) = G_CTTZ_ZERO_UNDEF %0(s32)
+    %3:_(s32) = nuw nsw G_ADD %1, %2
+    %5:_(s1) = G_ICMP intpred(eq), %0(s32), %4
+    %6:_(s32) = G_SELECT %5(s1), %4, %3
+    $v0 = COPY %6(s32)
+    RetRA implicit $v0
+
+...
+---
+name:            ffs_i64_expansion
+alignment:       4
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $a0, $a1
+
+    ; MIPS32-LABEL: name: ffs_i64_expansion
+    ; MIPS32: liveins: $a0, $a1
+    ; MIPS32: [[COPY:%[0-9]+]]:_(s32) = COPY $a0
+    ; MIPS32: [[COPY1:%[0-9]+]]:_(s32) = COPY $a1
+    ; MIPS32: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+    ; MIPS32: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; MIPS32: [[MV:%[0-9]+]]:_(s64) = G_MERGE_VALUES [[C1]](s32), [[C1]](s32)
+    ; MIPS32: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(eq), [[COPY]](s32), [[C1]]
+    ; MIPS32: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 -1
+    ; MIPS32: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY1]], [[C2]]
+    ; MIPS32: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY1]], [[C2]]
+    ; MIPS32: [[AND:%[0-9]+]]:_(s32) = G_AND [[XOR]], [[ADD]]
+    ; MIPS32: [[C3:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; MIPS32: [[CTLZ:%[0-9]+]]:_(s32) = G_CTLZ [[AND]](s32)
+    ; MIPS32: [[SUB:%[0-9]+]]:_(s32) = G_SUB [[C3]], [[CTLZ]]
+    ; MIPS32: [[ADD1:%[0-9]+]]:_(s32) = G_ADD [[SUB]], [[C3]]
+    ; MIPS32: [[XOR1:%[0-9]+]]:_(s32) = G_XOR [[COPY]], [[C2]]
+    ; MIPS32: [[ADD2:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[C2]]
+    ; MIPS32: [[AND1:%[0-9]+]]:_(s32) = G_AND [[XOR1]], [[ADD2]]
+    ; MIPS32: [[CTLZ1:%[0-9]+]]:_(s32) = G_CTLZ [[AND1]](s32)
+    ; MIPS32: [[SUB1:%[0-9]+]]:_(s32) = G_SUB [[C3]], [[CTLZ1]]
+    ; MIPS32: [[COPY2:%[0-9]+]]:_(s32) = COPY [[ICMP]](s32)
+    ; MIPS32: [[AND2:%[0-9]+]]:_(s32) = G_AND [[COPY2]], [[C]]
+    ; MIPS32: [[SELECT:%[0-9]+]]:_(s32) = G_SELECT [[AND2]](s32), [[ADD1]], [[SUB1]]
+    ; MIPS32: [[ADD3:%[0-9]+]]:_(s32) = G_ADD [[SELECT]], [[C]]
+    ; MIPS32: [[ICMP1:%[0-9]+]]:_(s32) = G_ICMP intpred(ult), [[ADD3]](s32), [[C]]
+    ; MIPS32: [[ADD4:%[0-9]+]]:_(s32) = G_ADD [[C1]], [[C1]]
+    ; MIPS32: [[COPY3:%[0-9]+]]:_(s32) = COPY [[ICMP1]](s32)
+    ; MIPS32: [[AND3:%[0-9]+]]:_(s32) = G_AND [[COPY3]], [[C]]
+    ; MIPS32: [[ADD5:%[0-9]+]]:_(s32) = G_ADD [[ADD4]], [[AND3]]
+    ; MIPS32: [[MV1:%[0-9]+]]:_(s64) = G_MERGE_VALUES [[ADD3]](s32), [[ADD5]](s32)
+    ; MIPS32: [[XOR2:%[0-9]+]]:_(s32) = G_XOR [[COPY]], [[C1]]
+    ; MIPS32: [[XOR3:%[0-9]+]]:_(s32) = G_XOR [[COPY1]], [[C1]]
+    ; MIPS32: [[OR:%[0-9]+]]:_(s32) = G_OR [[XOR2]], [[XOR3]]
+    ; MIPS32: [[ICMP2:%[0-9]+]]:_(s32) = G_ICMP intpred(eq), [[OR]](s32), [[C1]]
+    ; MIPS32: [[COPY4:%[0-9]+]]:_(s32) = COPY [[ICMP2]](s32)
+    ; MIPS32: [[AND4:%[0-9]+]]:_(s32) = G_AND [[COPY4]], [[C]]
+    ; MIPS32: [[SELECT1:%[0-9]+]]:_(s64) = G_SELECT [[AND4]](s32), [[MV]], [[MV1]]
+    ; MIPS32: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[SELECT1]](s64)
+    ; MIPS32: $v0 = COPY [[UV]](s32)
+    ; MIPS32: $v1 = COPY [[UV1]](s32)
+    ; MIPS32: RetRA implicit $v0, implicit $v1
+    %1:_(s32) = COPY $a0
+    %2:_(s32) = COPY $a1
+    %0:_(s64) = G_MERGE_VALUES %1(s32), %2(s32)
+    %4:_(s64) = G_CONSTANT i64 1
+    %6:_(s64) = G_CONSTANT i64 0
+    %3:_(s64) = G_CTTZ_ZERO_UNDEF %0(s64)
+    %5:_(s64) = nuw nsw G_ADD %3, %4
+    %7:_(s1) = G_ICMP intpred(eq), %0(s64), %6
+    %8:_(s64) = G_SELECT %7(s1), %6, %5
+    %9:_(s32), %10:_(s32) = G_UNMERGE_VALUES %8(s64)
+    $v0 = COPY %9(s32)
+    $v1 = COPY %10(s32)
+    RetRA implicit $v0, implicit $v1
+
+...

llvm/test/CodeGen/Mips/GlobalISel/llvm-ir/cttz.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 -mtriple=mipsel-linux-gnu -global-isel -verify-machineinstrs %s -o -| FileCheck %s -check-prefixes=MIPS32
+
+define i32 @cttz_i32(i32 %a) {
+; MIPS32-LABEL: cttz_i32:
+; MIPS32:       # %bb.0: # %entry
+; MIPS32-NEXT:    not $1, $4
+; MIPS32-NEXT:    addiu $2, $4, -1
+; MIPS32-NEXT:    and $1, $1, $2
+; MIPS32-NEXT:    ori $2, $zero, 32
+; MIPS32-NEXT:    clz $1, $1
+; MIPS32-NEXT:    subu $2, $2, $1
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+entry:
+  %0 = call i32 @llvm.cttz.i32(i32 %a, i1 false)
+  ret i32 %0
+}
+declare i32 @llvm.cttz.i32(i32, i1 immarg)
+
+define i64 @cttz_i64(i64  %a) {
+; MIPS32-LABEL: cttz_i64:
+; MIPS32:       # %bb.0: # %entry
+; MIPS32-NEXT:    ori $3, $zero, 0
+; MIPS32-NEXT:    sltiu $1, $4, 1
+; MIPS32-NEXT:    not $2, $5
+; MIPS32-NEXT:    addiu $5, $5, -1
+; MIPS32-NEXT:    and $2, $2, $5
+; MIPS32-NEXT:    ori $5, $zero, 32
+; MIPS32-NEXT:    clz $2, $2
+; MIPS32-NEXT:    subu $2, $5, $2
+; MIPS32-NEXT:    addiu $2, $2, 32
+; MIPS32-NEXT:    not $6, $4
+; MIPS32-NEXT:    addiu $4, $4, -1
+; MIPS32-NEXT:    and $4, $6, $4
+; MIPS32-NEXT:    clz $4, $4
+; MIPS32-NEXT:    subu $4, $5, $4
+; MIPS32-NEXT:    andi $1, $1, 1
+; MIPS32-NEXT:    movn $4, $2, $1
+; MIPS32-NEXT:    move $2, $4
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+entry:
+  %0 = call i64 @llvm.cttz.i64(i64 %a, i1 false)
+  ret i64 %0
+}
+declare i64 @llvm.cttz.i64(i64, i1 immarg)
+
+
+define i32 @ffs_i32_expansion(i32 %a) {
+; MIPS32-LABEL: ffs_i32_expansion:
+; MIPS32:       # %bb.0: # %entry
+; MIPS32-NEXT:    ori $1, $zero, 0
+; MIPS32-NEXT:    not $2, $4
+; MIPS32-NEXT:    addiu $3, $4, -1
+; MIPS32-NEXT:    and $2, $2, $3
+; MIPS32-NEXT:    ori $3, $zero, 32
+; MIPS32-NEXT:    clz $2, $2
+; MIPS32-NEXT:    subu $2, $3, $2
+; MIPS32-NEXT:    addiu $2, $2, 1
+; MIPS32-NEXT:    sltiu $3, $4, 1
+; MIPS32-NEXT:    andi $3, $3, 1
+; MIPS32-NEXT:    movn $2, $1, $3
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+entry:
+  %0 = call i32 @llvm.cttz.i32(i32 %a, i1 true)
+  %1 = add nuw nsw i32 %0, 1
+  %iszero = icmp eq i32 %a, 0
+  %ffs = select i1 %iszero, i32 0, i32 %1
+  ret i32 %ffs
+}
+
+define i64 @ffs_i64_expansion(i64 %a) {
+; MIPS32-LABEL: ffs_i64_expansion:
+; MIPS32:       # %bb.0: # %entry
+; MIPS32-NEXT:    ori $1, $zero, 1
+; MIPS32-NEXT:    ori $2, $zero, 0
+; MIPS32-NEXT:    sltiu $3, $4, 1
+; MIPS32-NEXT:    not $6, $5
+; MIPS32-NEXT:    addiu $7, $5, -1
+; MIPS32-NEXT:    and $6, $6, $7
+; MIPS32-NEXT:    ori $7, $zero, 32
+; MIPS32-NEXT:    clz $6, $6
+; MIPS32-NEXT:    subu $6, $7, $6
+; MIPS32-NEXT:    addiu $6, $6, 32
+; MIPS32-NEXT:    not $8, $4
+; MIPS32-NEXT:    addiu $9, $4, -1
+; MIPS32-NEXT:    and $8, $8, $9
+; MIPS32-NEXT:    clz $8, $8
+; MIPS32-NEXT:    subu $7, $7, $8
+; MIPS32-NEXT:    andi $3, $3, 1
+; MIPS32-NEXT:    movn $7, $6, $3
+; MIPS32-NEXT:    addiu $3, $7, 1
+; MIPS32-NEXT:    sltu $1, $3, $1
+; MIPS32-NEXT:    addiu $6, $2, 0
+; MIPS32-NEXT:    andi $1, $1, 1
+; MIPS32-NEXT:    addu $1, $6, $1
+; MIPS32-NEXT:    xori $4, $4, 0
+; MIPS32-NEXT:    xori $5, $5, 0
+; MIPS32-NEXT:    or $4, $4, $5
+; MIPS32-NEXT:    sltiu $4, $4, 1
+; MIPS32-NEXT:    andi $4, $4, 1
+; MIPS32-NEXT:    movn $3, $2, $4
+; MIPS32-NEXT:    movn $1, $2, $4
+; MIPS32-NEXT:    move $2, $3
+; MIPS32-NEXT:    move $3, $1
+; MIPS32-NEXT:    jr $ra
+; MIPS32-NEXT:    nop
+entry:
+  %0 = call i64 @llvm.cttz.i64(i64 %a, i1 true)
+  %1 = add nuw nsw i64 %0, 1
+  %iszero = icmp eq i64 %a, 0
+  %ffs = select i1 %iszero, i64 0, i64 %1
+  ret i64 %ffs
+}