[llvm-dev] Why does LLVM keep some loads in the loops even after applying the O3 optimization?

[Fami H via llvm-dev]

Is there any structure at the LLVM backend that holds this information? I
mean, can I get one machine instruction load and find out which other
instructions in this function or even in other functions may modify its
accessed memory location?

On Thu, Mar 28, 2019 at 6:18 PM Fami H <hoseini.f at gmail.com> wrote:

> Thank you all for your responses.
>
> On Thu, Mar 28, 2019 at 2:59 PM David Greene <dag at cray.com> wrote:
>
>> Ryan Taylor via llvm-dev <llvm-dev at lists.llvm.org> writes:
>>
>> > r0 gets overwritten inside the loop (assuming dst, src, src), is ldr
>> > r0, [r5] needed to initialize r0 for the loop at each iteration?
>>
>> Register allocation should handle that if the load is hoisted.
>>
>> I'm with the others.  The printf is the most likely culprit.
>>
>>                       -0David
>>
>> > On Thu, Mar 28, 2019 at 10:17 AM Fami H via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> >
>> >  Hello all,
>> >
>> >  I am looking at the assembly code of a loop body which is created by
>> applying O3 optimization. Here it is:
>> >
>> >  .LBB4_19:                               @ %for.body.91
>> >                                          @ =>This Inner Loop Header:
>> Depth=1
>> >          ldr r0, [r5]
>> >          mov r1, r8
>> >          add r0, r0, r7
>> >          vldr s0, [r0]
>> >          mov r0, r6
>> >          vcvt.f64.f32 d0, s0
>> >          vmov r2, r3, d0
>> >          bl fprintf
>> >          cmp r0, #0
>> >          blt .LBB4_25
>> >  @ BB#20:                                @ %for.cond.89
>> >                                          @   in Loop: Header=BB4_19
>> Depth=1
>> >          ldr r0, .LCPI4_2
>> >          add r4, r4, #1
>> >          add r7, r7, #4
>> >          ldr r0, [r0]
>> >          cmp r4, r0
>> >          blt .LBB4_19
>> >
>> >  There are no other basic blocks in the loop.  I am wondering why the
>> first load instruction (ldr r0, [r5]) is repeatedly executed in the loop
>> while the load
>> >  address (r5) is never changed in the loop body. Shouldn't this
>> instruction be moved out of the loop as a result of -licm flag? I mean this
>> load could have been
>> >  executed only once outside of the loop and the result could have been
>> saved in the register and used in the loop. I'd greatly appreciate if
>> anyone can tell me
>> >  why this is not the case.
>> >
>> >  Thank you in advance,
>> >  Fami
>> >
>> >
>> >
>> >  _______________________________________________
>> >  LLVM Developers mailing list
>> >  llvm-dev at lists.llvm.org
>> >  https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190329/bebd39ba/attachment.html>