On Jul 18, 2018, Richard Biener <richard.guent...@gmail.com> wrote: > On Wed, Jul 18, 2018 at 8:53 AM Alexandre Oliva <ol...@gnu.org> wrote: >> Instance discriminators are not compatible with LTO, in that the >> instance mapping is not preserved in LTO dumps. There are no plans to >> preserve discriminators in them.
> Because... ... it follows existing practice. BB discriminators are not saved for LTO either. They could be saved along with the CFG, but AFAICT they aren't. As for instance discriminators, we might be able to save them along with LOC information, but that would be quite wasteful, and because of the way ordinary maps are reconstructed when reading in the LTO data, we'd end up with yet another internal representation for line_maps. I was told there was no interest from our customers in using the converage and monitoring aspects of instance discriminators when performing link-time optimizations, and thus that it made sense to follow existing practice. I suspect there might be a way to assign instance discriminator numbers to individual function DECLs, and then walk up the lexical block tree to identify the DECL containing the block so as to obtain the discriminator number. This would be a lot less efficient, algorithmically speaking, but, provided that LTO dumps discriminator numbers as part of the decls, and enough info to reconstruct the lexical block trees, including the inlined-function enclosing blocks, that should work even for LTO, at least as long as different decls are maintained for each instance. Indeed, if this is the case, code ranges of lexical blocks in inlined subroutines would suffice to identify each instantiation, without the need for discriminators. It would be a lot more expensive to gather the information from that debug info than from discriminators, though. All this said, there doesn't seem to be much interest in that from Ada users to justify by itself the pursuit of yet another internal representation. I wonder if this sort of discriminator might be of interest for users of C++ templates as well. -- Alexandre Oliva, freedom fighter https://FSFLA.org/blogs/lxo Be the change, be Free! FSF Latin America board member GNU Toolchain Engineer Free Software Evangelist
