[LLVMdev] malloc / free & memcpy optimisations.

[Duncan Sands]

Hi Jeremy,

On 21/05/13 14:34, Jeremy Lakeman wrote:
>  > could you allocate the memory on the stack instead (alloca instruction)?
>
> This is mainly for string or binary blob handling, using the stack isn't a great
> idea for size reasons.
>
> While I'm experimenting with simple code examples now, and I picked a simple one
> for this email. I'm certain things will get much more complicated once I
> implement more features of the language.

the optimizer that does memcpy forwarding is in
   lib/Transforms/Scalar/MemCpyOptimizer.cpp
You might want to look into teaching it how to handle malloc'd memory and not
just alloca instructions.  I think the logic is in
   MemCpyOpt::performCallSlotOptzn

Ciao, Duncan.

>
>
> On Tue, May 21, 2013 at 8:45 PM, Jeremy Lakeman <Jeremy.Lakeman at gmail.com
> <mailto:Jeremy.Lakeman at gmail.com>> wrote:
>
>     The front end I'm building for an existing interpreted language is
>     unfortunately producing output similar to this far too often;
>
>     define void @foo(i8* nocapture %dest, i8* nocapture %src, i32 %len) nounwind {
>        %1 = tail call noalias i8* @malloc(i32 %len) nounwind
>        tail call void @llvm.memcpy.p0i8.p0i8.i32(i8* %1, i8* %src, i32 %len, i32
>     1, i1 false)
>        tail call void @llvm.memcpy.p0i8.p0i8.i32(i8* %dest, i8* %1, i32 %len,
>     i32 1, i1 false)
>        tail call void @free(i8* %1) nounwind
>        ret void
>     }
>
>     I'd like to be able to reduce this pattern to this;
>
>     define void @foo(i8* nocapture %dest, i8* nocapture %src, i32 %len) nounwind {
>        tail call void @llvm.memcpy.p0i8.p0i8.i32(i8* %dest, i8* %src, i32 %len,
>     i32 1, i1 false)
>        ret void
>     }
>
>     Optimising all cases of this pattern from within my front end's AST would be
>     difficult. I'd rather implement this as an llvm pass or two that runs after
>     other function passes have already cleaned up the mess I've made.
>
>     Has anyone written any passes to detect and combine multiple memory copies
>     that originated from the same data?
>     And then eliminate stores and malloc / free pairs for local pointers that
>     are never read from or captured?
>
>
>
>
>
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