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

via llvm-dev
2019-02-25T21:09:25Z

If you produced the .ll file with a command line along the lines of `clang –c
–O0 –emit-llvm …` then the IR functions will be annotated with the `optnone`
attribute. If you don't want that, a better command line would be `clang –c
–O2 –Xclang –disable-llvm-passes –emit-llvm …`
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Pedro
Olsen Ferreira via llvm-dev
Sent: Monday, February 25, 2019 6:28 AM
To: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Why is there still ineffective code after -o3
optimization?

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)"}

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