drreg losing track of spilled registers when called outside of insertion phase.

[hgreving2304]

When using drreg outside of the insertion phase, it defaults to a forward scan of the ilist to determine register liveness. At the beginning of the function drreg_forward_analysis, it flat-rate sets ever_spilled to false for each register. So if you call drreg_reserve_register() more than once, the ever_spilled of the first is false again. As a consequence, registers except the one of the last call of drreg_reserve_register() are never restored.

[johnfxgalea]

When and why would you use drreg_reserve_register outside the insertion phase?

[hgreving2304]

Using it in other phases is explicitly allowed (xref http://dynamorio.org/docs/page_drreg.html), except no optimization will be performed. For example the drreg-test has an example in instru2instru. I am not sure why instru2instru is important, but for example I currently need it for app2app, for an emulation sequence.

[derekbruening]

drreg use outside of the insertion phase will only work today if there is no other drreg use in that client or its libraries. See #3823.