[llvm-bugs] [Bug 44044] New: [MSP430][AVR][InstCombine][DAGCombine]Poor codegen for targets with no native shifts (7/8)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44044

            Bug ID: 44044
           Summary: [MSP430][AVR][InstCombine][DAGCombine]Poor codegen for
                    targets with no native shifts (7/8)
           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 testShiftAnd_1more( int x )  // (InstCombineCasts::transformZExtICmp)
{
  return x<0 ;
}

IR code:

define i16 @testShiftAnd_1more(i16 %x) {
entry:
  %x.lobit = lshr i16 %x, 15
  ret i16 %x.lobit
}

MSP430 Target code:

testShiftAnd_1more:
        swpb    r12
        mov.b   r12, r12
        clrc
        rrc     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        rra     r12
        ret

AVR Target code:

testShiftAnd_1more:
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        lsr     r25
        ror     r24
        ret


EXPECTED RESULT:

Source code:

int testShiftAnd_1more( int x )  // (InstCombineCasts::transformZExtICmp)
{
  return x<0 ;
}

Expected IR code:

define i16 @testShiftAnd_1more(i16 %x) {
entry:
  %cmp = icmp slt i16 %x, 0
  %conv = zext i1 %cmp to i16
  ret i16 %conv
}


Expected MSP430 Target code: 

testShiftAnd_1more:
        mov     r12, r13
        mov     #1, r12
        tst     r13
        jl      .LBB6_2
        clr     r12
.LBB6_2:
        ret


Expected AVR Target code:

testShiftAnd_1more:
        ldi     r18, 1
        tst     r25
        brmi    LBB6_2
        ldi     r18, 0
LBB6_2:
        mov     r24, r18
        clr     r25
        ret

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