On 7/26/23 07:08, Robin Dapp via Gcc-patches wrote:
So after thinking about it again - I'm still not really sure
I like treating every function as essentially an fesetround.
There is a reason why fesetround is special. Does LLVM behave
the same way?
But supposing we really, really want it and assuming there's consensus:
+ start_sequence ();
+ emit_insn (gen_frrmsi (DYNAMIC_FRM_RTL (cfun)));
+ rtx_insn *backup_insn = get_insns ();
+ end_sequence ();
A comment here would be nice why we need a sequence for a single
instruction. I'm not fully aware what insert_insn_end_basic_block
does but won't a
rtx_insn *last = BB_END (bb);
emit_insn_before_noloc (gen_frrmsi (DYNAMIC_FRM_RTL (cfun)), last, bb);
suffice? One way or another need these kinds of non-local
constructs here don't seem entirely rock solid.
Typically an LCM algorithm needs to insert on edges rather than at the
end of blocks -- this is particularly important to preserve its property
that on no path through the CFG can we have more evaluations of the
expression after PRE/LCM than before PRE/LCM.
The other thing edge insertions do is simplify the abnormal critical
edge problems. I'd have to dig into the precise details, but in the
generic PRE/LCM code we clobber the available expressions on critical
edges so that we don't try to hold a value live across that edge.
Thus the insertion point will tend to be the normal edge of a block that
ends with a call to a potentially throwing function.
Inserting on the edge also significantly simplifies handling of
conditional branches ;-)
Jeff
