[RFC] #pragma ivdep

[Redmond, Paul]

I have updated the patch to not add metadata on loads and stores where the pointer comes from an alloca. I wonder if the check should be more conservative and only include pointers coming from Arguments and GlobalValues (perhaps Constants too?)

paul



On 2013-03-05, at 1:38 PM, Redmond, Paul wrote:

> Hi,
>
> The attached patch implements support for #pragma ivdep in clang. The ivdep is represented in the AST using AttributedStmt as follows:
>
> |-AttributedStmt 0x5003538 <line:37:11, line:39:5>
> | |-IVDepAttr 0x5003520 <line:37:11>
> | `-ForStmt 0x50034e0 <line:38:3, line:39:5>
> |   |-BinaryOperator 0x50033d8 <line:38:8, col:12> 'int' '='
> |   | |-DeclRefExpr 0x5003390 <col:8> 'int' lvalue Var 0x50030e0 'i' 'int'
> |   | `-IntegerLiteral 0x50033b8 <col:12> 'int' 0
> |   |-<<<NULL>>>
> |   |-BinaryOperator 0x5003460 <col:15, col:19> 'int' '<'
> |   | |-ImplicitCastExpr 0x5003448 <col:15> 'int' <LValueToRValue>
> |   | | `-DeclRefExpr 0x5003400 <col:15> 'int' lvalue Var 0x50030e0 'i' 'int'
> |   | `-IntegerLiteral 0x5003428 <col:19> 'int' 10
> |   |-UnaryOperator 0x50034b0 <col:23, col:25> 'int' prefix '++'
> |   | `-DeclRefExpr 0x5003488 <col:25> 'int' lvalue Var 0x50030e0 'i' 'int'
> |   `-NullStmt 0x50034d0 <line:39:5>
>
> Code generation is handled by a new helper class CGLoopMetadata and a custom IRBuilder inserter CGBuilderInserter which can annotate instructions as they're inserted.
>
> The code generation is actually not correct. Right now any instruction with mayReadOrWriteMemory() inside a ivdep loop will get the llvm.mem.parallel_loop_access metadata. This is not correct in general as discussed here: http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-March/060096.html
>
> I was hoping someone with experience with clang codegen could suggest a good way to decide when to attach the llvm.mem metadata based on the requirements described in the llvmdev thread.. ?
>
> paul
>
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