[LLVMdev] PROPOSAL: IR representation of detailed struct assignment information (new version)

[Chris Lattner]

On Sep 6, 2012, at 4:24 PM, Dan Gohman <gohman at apple.com> wrote:

> Hello,
> 
> Persuant to feedback,
> 
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-August/052927.html
> 
> here is a new proposal for detailed struct assignment information.

Thanks Dan,

> Here's the example showing the basic problem:
> 
> struct bar {
> char x;
> float y;
> double z;
> };
> void copy_bar(struct bar *a, struct bar *b) {
> *a = *b;
> }
> 
> The solution I now propose here is to have front-ends describe the copy
> using metadata. For example:
> 
> call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %1, i64 16, i32 8, i1 false), !tbaa.struct !4
> […]
> !0 = metadata !{metadata !"Simple C/C++ TBAA"}
> !1 = metadata !{metadata !"omnipotent char", metadata !0}
> !2 = metadata !{metadata !"float", metadata !1}
> !3 = metadata !{metadata !"double", metadata !1}
> !4 = metadata !{metadata !5, i64 3, metadata !6, metadata !7}
> !5 = metadata !{i64 1, metadata !1}
> !6 = metadata !{i64 4, metadata !2}
> !7 = metadata !{i64 8, metadata !3}
> 
> Metadata nodes !0 through !3 are regular TBAA nodes as are already in use.
> 
> Metadata node !4 here is a top-level description of the memcpy. It holds a
> list of virtual members. An integer represents a padding field of that
> size. A metadata tuple represents an actual data field. The tuple's members
> are an integer size and a TBAA tag for the field.

How about just making "!4" be a list of triples, where the first two elements are integer offset/size, and the third entry is a TBAA pointer, or null for padding?  This would give us easier to read (and pretty print) llvm IR and may be more memory efficient as well.

-Chris