[Clang] Convergent Attribute

[David Majnemer via cfe-commits]

On Fri, May 6, 2016 at 2:36 PM, Richard Smith via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> On Fri, May 6, 2016 at 1:56 PM, Ettore Speziale via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> Hello,
>>
>> > In the case of foo, there could be a problem.
>> > If you do not mark it convergent, the LLVM sink pass push the call to
>> foo to the then branch of the ternary operator, hence the program has been
>> incorrectly optimized.
>> >
>> > Really? It looks like the problem is that you lied to the compiler by
>> marking the function as 'pure'. The barrier is a side-effect that cannot be
>> removed or duplicated, so it's not correct to mark this function as pure.
>>
>> I was trying to write a very small example to trick LLVM and trigger the
>> optimization. It is based on Transforms/Sink/convergent.ll:
>>
>> define i32 @foo(i1 %arg) {
>> entry:
>>   %c = call i32 @bar() readonly convergent
>>   br i1 %arg, label %then, label %end
>>
>> then:
>>   ret i32 %c
>>
>> end:
>>   ret i32 0
>> }
>>
>> declare i32 @bar() readonly convergent
>>
>
> This example looks wrong to me. It doesn't seem meaningful for a function
> to be both readonly and convergent, because convergent means the call has
> some side-effect visible to other threads and readonly means the call has
> no side-effects visible outside the function.
>
> Here is another example:
>>
>> void foo0(void);
>> void foo1(void);
>>
>> __attribute__((convergent)) void baz() {
>>   barrier(CLK_GLOBAL_MEM_FENCE);
>> }
>>
>> void bar(int x, global int *y) {
>>   if (x < 5)
>>     foo0();
>>   else
>>     foo1();
>>
>>   baz();
>>
>>   if (x < 5)
>>     foo0();
>>   else
>>     foo1();
>> }
>>
>
> This one looks a lot more interesting. It looks like 'convergent' is a way
> of informing LLVM that the call cannot be duplicated, yes? That being the
> case, how is this attribute different from the existing
> [[clang::noduplicate]] / __attribute__((noduplicate)) attribute?
>

I think it has more to do with LLVM's definition of convergent: that you
really do not want control dependencies changing for a callsite.

http://llvm.org/docs/LangRef.html#function-attributes


>
> Based on Transforms/JumpThreading/basic.ll:
>>
>> define void @h_con(i32 %p) {
>>   %x = icmp ult i32 %p, 5
>>   br i1 %x, label %l1, label %l2
>>
>> l1:
>>   call void @j()
>>   br label %l3
>>
>> l2:
>>   call void @k()
>>   br label %l3
>>
>> l3:
>> ; CHECK: call void @g() [[CON:#[0-9]+]]
>> ; CHECK-NOT: call void @g() [[CON]]
>>   call void @g() convergent
>>   %y = icmp ult i32 %p, 5
>>   br i1 %y, label %l4, label %l5
>>
>> l4:
>>   call void @j()
>>   ret void
>>
>> l5:
>>   call void @k()
>>   ret void
>> ; CHECK: }
>> }
>>
>> If you do not mark baz convergent, you get this:
>>
>> clang -x cl -emit-llvm -S -o - test.c -O0 | opt -mem2reg -jump-threading
>> -S
>>
>> define void @bar(i32 %x) #0 {
>> entry:
>>   %cmp = icmp slt i32 %x, 5
>>   br i1 %cmp, label %if.then2, label %if.else3
>>
>> if.then2:                                         ; preds = %entry
>>   call void @foo0()
>>   call void @baz()
>>   call void @foo0()
>>   br label %if.end4
>>
>> if.else3:                                         ; preds = %entry
>>   call void @foo1()
>>   call void @baz()
>>   call void @foo1()
>>   br label %if.end4
>>
>> if.end4:                                          ; preds = %if.else3,
>> %if.then2
>>   ret void
>> }
>>
>> Which is illegal, as the value of x might not be the same for all
>> work-items.
>>
>> I’ll update the patch such as:
>>
>> * it uses the example about jump-threading
>> * it marks the attribute available in OpenCL/Cuda
>> * it provides the [[clang::convergent]] attribute
>>
>> Thanks,
>> Ettore Speziale
>>
>> --------------------------------------------------
>> Ettore Speziale — Compiler Engineer
>> speziale.ettore at gmail.com
>> espeziale at apple.com
>> --------------------------------------------------
>>
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>> cfe-commits at lists.llvm.org
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>>
>
>
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