On 2017-03-27 12:18:37 -0400, Tom Lane wrote: > I wrote: > > As to the point of whether it actually helps or not ... > > on gcc 4.4.7 (RHEL 6), it makes things *WORSE*. We go from about half of > > the dispatches getting routed through a common location, to *all* of them > > (except one; for some odd reason the first EEO_NEXT in EEOP_NULLIF > > survives as a separate jump). This seems like a bug, but there it is. > > So after a bit of googling, this is a very longstanding complaint: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39284 > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71785 > > (hm, think I know the second submitter)
I don't think that's precisely the same issue - as long as some of the goto branches survive, we're not hitting the full brunt of the compgoto thing. I think we're essentially avoiding some of that because we're "precomputing" the dispatch_table lookup. To count the number of jumps I used: gdb -batch -ex 'disassemble/s ExecInterpExpr' execExprInterp.o|grep jmpq|grep -v ExecInterpExpr|wc -l which'll only include indirect jumps (since otherwise jumps will look like "jmpq 0x35f2 <ExecInterpExpr+2066>") standard flags -fno-crossjumping gcc-5 (5.4.1-8): 34 82 gcc-6 (6.3.0-8): 34 82 gcc-7 (7.0.1): 71 108 gcc-snapshot: 72 108 So that doesn't look too bad. > My feeling at this point is that we might be better off disabling > the computed-goto case by default. At the very least, we're going > to need a version check that restricts it to latest gcc. In my measurements it's still faster in at least gcc-5/6, even without the option (largely because it avoids array bounds checks on the jump table built for the switch). Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers