[LLVMdev] DebugInfo and Metadata Store

[Dan Gohman]

Hi Devang,

> +  WeakVH DbgInfo;

This holds an MDNode* which contains information which is used
to look up a DebugLoc? It sounds like there's an unnecessary layer
of indirection here. Would it make sense to make Instruction just
have a DebugLoc member instead? I don't know what that would mean
for the getMD API, but my understanding of the Metadata design
was that things like nsw, tail, and align could eventually be
considered Metadata, so perhaps there's a way to handle this.

Also, please mention the issue of increasing the size of
Instruction for non-debug compilation and indicate what the
plan is.

A few misc comments:

> -  return MDHandlerNames[Name] = Count + 1;
> +  return MDHandlerNames[Name] = Count + 1 + 1;

This code is non-obvious enough to justify a comment.

> -    // MD Handlers are numbered from 1.
> +    // MD Handlers are numbered from 1. 

Please don't add trailing whitespace, especially to lines which
aren't otherwise changed.

> +#include "llvm/Support/Timer.h"
...
> +  Timer *MDTimer;

This isn't related to the rest of the patch.

Dan

On Nov 25, 2009, at 10:41 AM, Devang Patel wrote:

> Hi All,
> 
> Now, in llvm trunk we support custom metadata, which can be attached
> with llvm instructions. One of the user is "debugging information".
> This user is special (not just because it is in llvm svn tree:)
> because whenever debug info is available, it is likely that
> corresponding metadata is attached with almost all instructions. In
> other words, usually it is all or nothing. This means, using Metadata
> Store to access debugging information attached with each llvm
> instruction is expensive, in terms of compile time. If debugging
> information is directly stored inside llvm::Instruction then it speeds
> up compile time. In my measurements this speeds up LLVMContext.cpp
> compile time (-O0 -g) by almost 10%. I do not see any strong reason to
> give up this gain.
> 
> Thoughts ? I am attaching tentative patch with this email.
> -
> Devang
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