[llvm-dev] How to use CostModel?

[Stephen Thomas]

Nadav,

Thanks for your help. I tried adding multiplication, but I'm still seeing
0s and 1s:

$ cat test.c
int main(){
    int r = rand();
    if (r > 1){
        return r*8;
    }
    return r*86;
}

$ clang-3.6 -S -emit-llvm test.c -o test.bc

$ opt-3.6   -analyze  -targetlibinfo -cost-model ./test.bc
Printing analysis 'Target Library Information':
Pass::print not implemented for pass: 'Target Library Information'!
Printing analysis 'Cost Model Analysis' for function 'main':
Cost Model: Unknown cost for instruction:   %1 = alloca i32, align 4
Cost Model: Unknown cost for instruction:   %r = alloca i32, align 4
Cost Model: Found an estimated cost of 1 for instruction:   store i32 0,
i32* %1
Cost Model: Unknown cost for instruction:   %2 = call i32 (...)* @rand()
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %2,
i32* %r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %3 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %4 = icmp sgt
i32 %3, 1
Cost Model: Found an estimated cost of 0 for instruction:   br i1 %4, label
%5, label %8
Cost Model: Found an estimated cost of 1 for instruction:   %6 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %7 = mul nsw
i32 %6, 8
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %7,
i32* %1
Cost Model: Found an estimated cost of 0 for instruction:   br label %11
Cost Model: Found an estimated cost of 1 for instruction:   %9 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %10 = mul nsw
i32 %9, 86
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %10,
i32* %1
Cost Model: Found an estimated cost of 0 for instruction:   br label %11
Cost Model: Found an estimated cost of 1 for instruction:   %12 = load i32*
%1
Cost Model: Found an estimated cost of 0 for instruction:   ret i32 %12


I also tried adding setting the target with the flags you specified, but
that didn't seem to change anything:

$ opt-3.6   -analyze -mcpu=core-avx2 -mtriple=x86_64-apple-darwin
 -targetlibinfo -cost-model ./test.bc
Printing analysis 'Target Library Information':
Pass::print not implemented for pass: 'Target Library Information'!
Printing analysis 'Cost Model Analysis' for function 'main':
Cost Model: Unknown cost for instruction:   %1 = alloca i32, align 4
Cost Model: Unknown cost for instruction:   %r = alloca i32, align 4
Cost Model: Found an estimated cost of 1 for instruction:   store i32 0,
i32* %1
Cost Model: Unknown cost for instruction:   %2 = call i32 (...)* @rand()
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %2,
i32* %r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %3 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %4 = icmp sgt
i32 %3, 1
Cost Model: Found an estimated cost of 0 for instruction:   br i1 %4, label
%5, label %8
Cost Model: Found an estimated cost of 1 for instruction:   %6 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %7 = mul nsw
i32 %6, 8
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %7,
i32* %1
Cost Model: Found an estimated cost of 0 for instruction:   br label %11
Cost Model: Found an estimated cost of 1 for instruction:   %9 = load i32*
%r, align 4
Cost Model: Found an estimated cost of 1 for instruction:   %10 = mul nsw
i32 %9, 86
Cost Model: Found an estimated cost of 1 for instruction:   store i32 %10,
i32* %1
Cost Model: Found an estimated cost of 0 for instruction:   br label %11
Cost Model: Found an estimated cost of 1 for instruction:   %12 = load i32*
%1
Cost Model: Found an estimated cost of 0 for instruction:   ret i32 %12


Any other ideas?


Thanks,
Steve

On Wed, Aug 5, 2015 at 1:10 PM, Nadav Rotem <nrotem at apple.com> wrote:

>
> On Aug 5, 2015, at 7:54 AM, Stephen Thomas <
> stephen.warner.thomas at gmail.com> wrote:
>
> Sorry if double posted... might have sent this to old mailing list
> address...
>
> ---------- Forwarded message ----------
> From: Stephen Thomas <stephen.warner.thomas at gmail.com>
> Date: Wed, Aug 5, 2015 at 10:39 AM
> Subject: How to use CostModel?
> To: LLVM Dev <llvmdev at cs.uiuc.edu>
>
>
> Hi,
>
> I'm trying to use the built-in CostModel class to estimate the number of
> machine instructions for each IR Instruction. To get a feel for what kind
> of output the CostModel pass gives, I ran it on the command line like so:
>
> $ cat test.c
> int main(){
>     int r = rand();
>     if (r > 1){
>         return ++r;
>     }
>     return r*86;
> }
>
> $ clang-3.6 -S -emit-llvm test.c -o test.bc
>
> $ opt-3.6  -analyze -targetlibinfo -cost-model ./test.bc
> Printing analysis 'Target Library Information':
> Pass::print not implemented for pass: 'Target Library Information'!
> Printing analysis 'Cost Model Analysis' for function 'main':
> Cost Model: Unknown cost for instruction:   %1 = alloca i32, align 4
> Cost Model: Unknown cost for instruction:   %r = alloca i32, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   store i32 0,
> i32* %1
> Cost Model: Unknown cost for instruction:   %2 = call i32 (...)* @rand()
> Cost Model: Found an estimated cost of 1 for instruction:   store i32 %2,
> i32* %r, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   %3 = load i32*
> %r, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   %4 = icmp sgt
> i32 %3, 1
> Cost Model: Found an estimated cost of 0 for instruction:   br i1 %4,
> label %5, label %8
> Cost Model: Found an estimated cost of 1 for instruction:   %6 = load i32*
> %r, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   %7 = add nsw
> i32 %6, 1
> Cost Model: Found an estimated cost of 1 for instruction:   store i32 %7,
> i32* %r, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   store i32 %7,
> i32* %1
> Cost Model: Found an estimated cost of 0 for instruction:   br label %11
> Cost Model: Found an estimated cost of 1 for instruction:   %9 = load i32*
> %r, align 4
> Cost Model: Found an estimated cost of 1 for instruction:   %10 = mul nsw
> i32 %9, 86
> Cost Model: Found an estimated cost of 1 for instruction:   store i32 %10,
> i32* %1
> Cost Model: Found an estimated cost of 0 for instruction:   br label %11
> Cost Model: Found an estimated cost of 1 for instruction:   %12 = load
> i32* %1
> Cost Model: Found an estimated cost of 0 for instruction:   ret i32 %12
>
>
> Is it expected to only have 1s and 0s output?
>
>
> The output looks correct. Try adding multiplication and division and
> you'll see higher costs.
>
> Also, what target architecture is being used for the estimations? (Is it
> auto detecting the architecture I'm on?)
>
>
> When the cost model analysis uses it uses the target that the compiler
> driver sets.  In your experiments with OPT you can request a specific
> target using the -mcpu and -mtriple flags. Like this:
>
> -mcpu=core-avx2 -mtriple=x86_64-apple-darwin
>
> You can find more examples under llvm/test/Analysis/CostModel.
>
> -Nadav
>
>
>
> Thanks,
> Steve
>
> _______________________________________________
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>
>
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