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[PR 18143] thunk range
- From: Nathan Sidwell <nathan at codesourcery dot com>
- To: gcc <gcc at gcc dot gnu dot org>
- Date: Mon, 01 Nov 2004 10:14:45 +0000
- Subject: [PR 18143] thunk range
- Organization: Codesourcery LLC
PR 18143 is about a problem with the newly restricted thunk range.
Option 1 is to revert the restriction.
Option 2 is to add a sorry.
Option 1 is unarguably to be preferred, if it has no down sides.
Unfortunately it does. Reverting the patch makes lang_decl bigger, and
that impacts compile time -- by a surprising amount. I built
--disable-checking compilers on both powerpc-apple-darwin and
i686-pc-linux-gnu and then measured the -O0 compile time for QT. There
was a 1% slowdown on both platforms. This is bad.
Either the size of lang_decl just 'fell off a cliff' performance wise
with this increase, or the size/time dependency is reasonably linear
at this size. If it's the latter, then this is good data that shrinking
data structures gets us _good_ time wins.
There is an Option1.5, in that there are still 3 spare bits, so we could
increase the size limit (and allow 2MB objects) without time penalty.
Alternatives like an on-the-side hash table for this kind of auxiliary
structure are more work than I'd want to do right now at this stage.
comments?
nathan
--
Nathan Sidwell :: http://www.codesourcery.com :: CodeSourcery LLC
nathan@codesourcery.com :: http://www.planetfall.pwp.blueyonder.co.uk