[llvm-dev] Why is there still ineffective code after -o3 optimization?

Pedro Olsen Ferreira via llvm-dev
2019-02-25T14:27:36Z

You're asking LLVM not to optimise that function:


define void @_Z27entityMainDataInputCallbackdRd(double %input, double*
dereferenceable(8) %o0) local_unnamed_addr #0

where the attribute list is defined as:

attributes #0 = { noinline nounwind optnone uwtable



Removing "optnone" yields what you'd expect.



On 25/02/2019 14:09, alexp via llvm-dev wrote:

Hi,

I have some IR module from random generation (mostly ineffective
instructions).
It has a function with void return, and two function arguments where one
is a reference.
Therefore, I expect every instruction not altering the value at the 2nd
arguments address should be ineffective.

Here is the function definition (see below for full ll):

define void @_Z27entityMainDataInputCallbackdRd(double %input, double*
dereferenceable(8) %o0) local_unnamed_addr #0 {

The ll below has been optimized using opt -O3 -S input.ll -o out.ll
The cfg is plotted as png here:
https://ibb.co/f1B0rnm

My question is: why is there still some unnecessary 'lint' in the
optimization result?
E.G. from the splittedblock19 onwards:
there is one effective instruction
store double 0x3FF787378698A951, double* %o0

everything what follows (conditional branch, splittedblock38, 21, 42)
could safely be removed (just leaving a 'ret void' after the store
instruction).

Why is this not removed?

Is there a pass (combination) which could remove all the ineffective
instructions?

Thanks
Alex




//=====================================================//=== full ll below



; ModuleID = '0.1.800866.ll'
source_filename = "entityMain.cpp"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: noinline nounwind optnone uwtable
define void @_Z27entityMainDataInputCallbackdRd(double %input, double*
dereferenceable(8) %o0) local_unnamed_addr #0 {
entry:
  %input.addr = alloca double, align 8
  %o0.addr = alloca double*, align 8
  %ui = alloca i32, align 4
  store double %input, double* %input.addr, align 8
  %0 = fptoui double %input to i64
  %1 = fptrunc double %input to float
  %2 = mul i64 %0, %0
  %3 = fptrunc double %input to float
  %4 = mul i64 %0, %0
  %5 = fsub float %3, %3
  store double* %o0, double** %o0.addr
  %6 = and i64 %0, %0
  %7 = fptoui float %3 to i1
  %8 = ashr i1 %7, %7
  store double* null, double** %o0.addr
  %9 = fptosi double %input to i64
  %10 = shl i1 %7, %7
  %11 = sitofp i1 %8 to float
  %12 = icmp uge i64 %6, %4
  %13 = and i64 %0, %6
  %14 = fcmp ole float %11, %11
  %15 = ashr i1 %10, %7
  %16 = sext i1 %10 to i64
  %17 = ashr i1 %7, %12
  %18 = alloca [2 x double]
  %19 = fcmp true float %11, %3
  br label %splittedblock7

splittedblock7:                                   ; preds = %entry
  %20 = select i1 %8, i1 %17, i1 %19
  %21 = fptoui float %1 to i1
  br label %splittedblock1

splittedblock1:                                   ; preds = %splittedblock7
  %22 = fptrunc double %input to float
  %23 = shl i1 %17, %20
  %24 = fsub float %5, %5
  br label %splittedblock35

splittedblock36:                                  ; preds = %splittedblock10
  br label %splittedblock15

splittedblock38:                                  ; preds = %splittedblock19
  br label %splittedblock21

splittedblock10:                                  ; preds = %splittedblock35
  br label %splittedblock36

splittedblock35:                                  ; preds = %splittedblock1
  br i1 %12, label %splittedblock14, label %splittedblock10

splittedblock14:                                  ; preds = %splittedblock35
  br label %splittedblock19

splittedblock19:                                  ; preds = %splittedblock14
  store double 0x3FF787378698A951, double* %o0
  br i1 %23, label %splittedblock38, label %splittedblock42

splittedblock42:                                  ; preds %splittedblock21,
%splittedblock19
  ret void

splittedblock15:                                  ; preds = %splittedblock36
  ret void

splittedblock21:                                  ; preds = %splittedblock38
  %25 = xor i64 %6, %4
  br label %splittedblock42
}

; Function Attrs: norecurse nounwind readnone
define <2 x double> @func_3701811888700380719897139(...)
local_unnamed_addr #1 {
entry:
  ret <2 x double> <double 0xC00934F9D7726A00, double
0xFFCFDA8E9AFFB51D>
}

; Function Attrs: norecurse noreturn nounwind readnone
define { double, double } @func_0065559471331315590586(...)
local_unnamed_addr #2 {
entry:
  br label %splittedblock1

splittedblock1:                                   ; preds = %entry,
%splittedblock1
  br label %splittedblock1
}

attributes #0 = { noinline nounwind optnone uwtable
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false"
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false"
"no-jump-tables"="false"
"no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
"no-trapping-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = { norecurse nounwind readnone }
attributes #2 = { norecurse noreturn nounwind readnone }

!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{!"clang version 7.0.1 (branches/release_70 348238)"}
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190225/0b029668/attachment.html>