[X86] Enable 8-bit SHL to convert to LEA

[RKSimon]

Do you know why this wasn't done before? It seems odd that i16 support was added but not i8

[craig.topper]

We used to not do any 8-bit ops. 8-bit add was added by Sanjay within the last few months. So I guess no one had gotten around to it?

[RKSimon]

OK, LGTM

lib/Target/X86/X86InstrInfo.cpp

MachineInstr *InsMI =
       BuildMI(*MFI, MBBI, MI.getDebugLoc(), get(TargetOpcode::COPY))
           .addReg(InRegLEA, RegState::Define, SubReg)
           .addReg(Src, getKillRegState(IsKill));
 
   MachineInstrBuilder MIB =
       BuildMI(*MFI, MBBI, MI.getDebugLoc(), get(Opcode), OutRegLEA);
   switch (MIOpc) {
   default: llvm_unreachable("Unreachable!");
+  case X86::SHL8ri:
   case X86::SHL16ri: {
     unsigned ShAmt = MI.getOperand(2).getImm();
     MIB.addReg(0).addImm(1ULL << ShAmt)
        .addReg(InRegLEA, RegState::Kill).addImm(0).addReg(0);
     break;
   }
   case X86::INC8r:
   case X86::INC16r:
MachineInstrBuilder MIB =
             .addImm(0)
             .addReg(0);
     if (ImplicitOp.getReg() != 0)
       MIB.add(ImplicitOp);
     NewMI = MIB;
 
     break;
   }
+  case X86::SHL8ri:
+    Is8BitOp = true;
+    LLVM_FALLTHROUGH;
   case X86::SHL16ri: {
     assert(MI.getNumOperands() >= 3 && "Unknown shift instruction!");
     unsigned ShAmt = getTruncatedShiftCount(MI, 2);
     if (!isTruncatedShiftCountForLEA(ShAmt))
       return nullptr;
     return convertToThreeAddressWithLEA(MIOpc, MFI, MI, LV, Is8BitOp);
   }
   case X86::INC64r:

lib/Target/X86/X86InstrShiftRotate.td

 def SHL32rCL : I<0xD3, MRM4r, (outs GR32:$dst), (ins GR32:$src1),
                  "shl{l}\t{%cl, $dst|$dst, cl}",
                  [(set GR32:$dst, (shl GR32:$src1, CL))]>, OpSize32;
 def SHL64rCL : RI<0xD3, MRM4r, (outs GR64:$dst), (ins GR64:$src1),
                   "shl{q}\t{%cl, $dst|$dst, cl}",
                   [(set GR64:$dst, (shl GR64:$src1, CL))]>;
 } // Uses = [CL], SchedRW
 
+let isConvertibleToThreeAddress = 1 in {   // Can transform into LEA.
 def SHL8ri   : Ii8<0xC0, MRM4r, (outs GR8 :$dst), (ins GR8 :$src1, u8imm:$src2),
                    "shl{b}\t{$src2, $dst|$dst, $src2}",
                    [(set GR8:$dst, (shl GR8:$src1, (i8 imm:$src2)))]>;
 
-let isConvertibleToThreeAddress = 1 in {   // Can transform into LEA.
 def SHL16ri  : Ii8<0xC1, MRM4r, (outs GR16:$dst), (ins GR16:$src1, u8imm:$src2),
                    "shl{w}\t{$src2, $dst|$dst, $src2}",
                    [(set GR16:$dst, (shl GR16:$src1, (i8 imm:$src2)))]>,
                    OpSize16;
 def SHL32ri  : Ii8<0xC1, MRM4r, (outs GR32:$dst), (ins GR32:$src1, u8imm:$src2),
                    "shl{l}\t{$src2, $dst|$dst, $src2}",
                    [(set GR32:$dst, (shl GR32:$src1, (i8 imm:$src2)))]>,
                    OpSize32;

test/CodeGen/X86/fast-isel-shift.ll

 ; CHECK-NEXT:    retq
   %c = ashr i64 %a, %b
   ret i64 %c
 }
 
 define i8 @shl_imm1_i8(i8 %a) {
 ; CHECK-LABEL: shl_imm1_i8:
 ; CHECK:       ## %bb.0:
-; CHECK-NEXT:    movl %edi, %eax
-; CHECK-NEXT:    shlb $1, %al
+; CHECK-NEXT:    ## kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    leal (,%rdi,2), %eax
 ; CHECK-NEXT:    ## kill: def $al killed $al killed $eax
 ; CHECK-NEXT:    retq
   %c = shl i8 %a, 1
   ret i8 %c
 }
 
 define i16 @shl_imm1_i16(i16 %a) {
 ; CHECK-LABEL: shl_imm1_i16:

test/CodeGen/X86/mul-constant-i8.ll

 ; X64-NEXT:    retq
   %m = mul i8 %x, 3
   ret i8 %m
 }
 
 define i8 @test_mul_by_4(i8 %x) {
 ; X64-LABEL: test_mul_by_4:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    shlb $2, %al
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    leal (,%rdi,4), %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
   %m = mul i8 %x, 4
   ret i8 %m
 }
 
 define i8 @test_mul_by_5(i8 %x) {
 ; X64-LABEL: test_mul_by_5:

 ; X64-NEXT:    retq
   %m = mul i8 %x, 7
   ret i8 %m
 }
 
 define i8 @test_mul_by_8(i8 %x) {
 ; X64-LABEL: test_mul_by_8:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    shlb $3, %al
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    leal (,%rdi,8), %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
   %m = mul i8 %x, 8
   ret i8 %m
 }
 
 define i8 @test_mul_by_9(i8 %x) {
 ; X64-LABEL: test_mul_by_9:

 ; X64-NEXT:    retq
   %m = mul i8 %x, 73
   ret i8 %m
 }
 
 define i8 @test_mul_by_520(i8 %x) {
 ; X64-LABEL: test_mul_by_520:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    shlb $3, %al
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    leal (,%rdi,8), %eax
 ; X64-NEXT:    # kill: def $al killed $al killed $eax
 ; X64-NEXT:    retq
   %m = mul i8 %x, 520
   ret i8 %m
 }
 
 define i8 @test_mul_by_neg10(i8 %x) {
 ; X64-LABEL: test_mul_by_neg10: