[cfe-commits] [PATCH] Invariants (and Assume Aligned) - Clang

[Tobias Grosser]

On 12/05/2012 11:45 PM, Hal Finkel wrote:
> ----- Original Message -----
>> From: "Tobias Grosser"<tobias at grosser.es>
>> To: "Hal Finkel"<hfinkel at anl.gov>
>> Cc: cfe-commits at cs.uiuc.edu, reviews+D149+public+707ba5224a940551 at llvm-reviews.chandlerc.com
>> Sent: Wednesday, December 5, 2012 4:37:01 PM
>> Subject: Re: [cfe-commits] [PATCH] Invariants (and Assume Aligned) - Clang
>>
>> On 12/05/2012 11:29 PM, Hal Finkel wrote:
>>> ----- Original Message -----
>>>> From: "Tobias Grosser"<tobias at grosser.es>
>>>> To:
>>>> reviews+D149+public+707ba5224a940551 at llvm-reviews.chandlerc.com
>>>> Cc: hfinkel at anl.gov, cfe-commits at cs.uiuc.edu
>>>> Sent: Wednesday, December 5, 2012 4:04:54 PM
>>>> Subject: Re: [cfe-commits] [PATCH] Invariants (and Assume Aligned)
>>>> - Clang
>>>>
>>>> On 12/05/2012 09:39 PM, hfinkel at anl.gov wrote:
>>>>>      Test cases have been enhanced. We now use @llvm.invariant.
>>>>
>>>>> Index: test/CodeGen/builtin-assume.c
>>>>> ===================================================================
>>>>> --- /dev/null
>>>>> +++ test/CodeGen/builtin-assume.c
>>>>> @@ -0,0 +1,9 @@
>>>>> +// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -o
>>>>> -
>>>>> %s | FileCheck %s
>>>>> +
>>>>> +// CHECK: @test1
>>>>> +int test1(int *a) {
>>>>> +// CHECK: call void @llvm.invariant(i1
>>>>> +  __builtin_assume(a != 0);
>>>>> +  return a[0];
>>>>
>>>> The second check line looks incomplete? Is this intended?
>>>
>>> Yes. I did not want to match the variable name specifying the
>>> condition.
>>>
>>> [For some reason, I can't see this comment in the web interface].
>>
>> I have no account for the web interface, but hoped the emails would
>> show
>> up there anyway. Is this not the case?
>
> It seems not.
>
>>
>> Would you mind adding the entire test case (with some regexp magic)
>> or
>> in case this is too much work can you add an example to the
>> documentation?
>
> Sure. I'll try harder ;)
>
>> Does __builtin_assume() basically take an i1 as an
>> argument and allows an arbitrary set of instructions that lead to
>> this
>> i1. Can I have something, like
>>
>> __builtin_assume(clamp(0, x, 100)>= 0);
>> __builtin_assume(clamp(0, x, 100)<= 100);
>>
>> to pass information about the behavior of the function clamp?
>
> Yes; that's the idea. Currently, the only infrastructure I have is to apply this information to pointer alignments. In the future, the idea is to integrate this with ScalarEvolution, etc.

I will be the first user. It is perfect to pass information about the 
possible values of parameters.

I can see something like

__builtin_assume(B > 0 && B < 100);

or

__builtin_assume(size == 8 || size == 16 || size == 32);

Very helpful. Thanks a lot for making this available.

Tobi