[Bug rtl-optimization/64317] [5 Regression] Ineffective allocation of PIC base register

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64317

--- Comment #19 from Jeffrey A. Law <law at redhat dot com> ---
Making reload_cse_regs less pessimistic at CODE_LABELs really isn't feasible. 
cselib isn't easily turned into a scoped hash table and the multiple assignment
nature of RTL means that a simple scoped hash table isn't sufficient anyway.

I'm currently poking at postreload-gcse.  The basic idea being to detect when
an occurrence is transparent through a block and an occurrence exists in the
block's one and only predecessor.  In that case pretend the occurrence also
exists at the end of the current block.  I don't expect this to occur often,
thus I'm not computing transparency for each expression in all blocks and doing
a traditional dataflow propagation.

With that special case in place, I get just 5 reloads of the PIC register,
which appears minimal given the other register allocation decisions.  I need to
ponder a couple implementation details, but this may be the final resolution to
this BZ :-)