[llvm-bugs] [Bug 44040] New: [MSP430][AVR][InstCombine][DAGCombine]Poor codegen for targets with no native shifts (3/8)

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https://bugs.llvm.org/show_bug.cgi?id=44040

            Bug ID: 44040
           Summary: [MSP430][AVR][InstCombine][DAGCombine]Poor codegen for
                    targets with no native shifts (3/8)
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: joan.lluch at icloud.com
                CC: llvm-bugs at lists.llvm.org

A number of comparisons involving bit tests are converted into shifts by
InstCombine and DAGCombine. However, shifts are expensive for most 8 and 16 bit
targets with comparatively cheaper selects. 

It is desirable that selects are emitted instead of shifts for these targets.
The following cases were identified in TargetLowering and DAGCombine and were
fixed by:

https://reviews.llvm.org/D69116 
https://reviews.llvm.org/D69120
https://reviews.llvm.org/D69326
https://reviews.llvm.org/D70042

Cases in InstCombine remain to be fixed. In llvm-dev it has been suggested that
these cases should be fixed by reversing the current canonicalisation. I am
showing them in this and following reports:

REPORTED CASE:

Source code:

int testExtendSignBit_0( int x )  // (InstCombineCasts::transformSExtICmp)
{
  return x<0 ? 0 : -1;
}

IR code:

define i16 @testExtendSignBit_0(i16 %x) {
entry:
  %x.lobit = ashr i16 %x, 15
  %x.lobit.not = xor i16 %x.lobit, -1
  ret i16 %x.lobit.not
}

MSP430 Target code:

testExtendSignBit_0:
        swpb    r12
        sxt     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        inv     r12
        ret

AVR Target code:

testExtendSignBit_0:
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        asr     r25
        ror     r24
        com     r24
        com     r25
        ret

EXPECTED RESULT:

Source code:

int testExtendSignBit_0( int x )  // (InstCombineCasts::transformSExtICmp)
{
  return x<0 ? 0 : -1;
}

Expected IR code:

define i16 @testExtendSignBit_0(i16 %x) {
entry:
  %cmp = icmp sgt i16 %x, -1
  %cond = sext i1 %cmp to i16
  ret i16 %cond
}


Expected MSP430 Target code: 

testExtendSignBit_0:
        mov     r12, r13
        mov     #-1, r12
        tst     r13
        jge     .LBB2_2
        clr     r12
.LBB2_2:
        ret


Expected AVR Target code:

testExtendSignBit_0:
        tst     r25
        brpl    LBB2_2
        ldi     r24, 0
        ldi     r25, 0
        ret
LBB2_2:
        ldi     r24, 255
        ldi     r25, 255
        ret

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