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Re: [tree-ssa] More CCP fixes [patch]

From: Mark Mitchell <mark at codesourcery dot com>
To: Neil Booth <neil at daikokuya dot co dot uk>, Fergus Henderson <fjh at cs dot mu dot OZ dot AU>
Cc: Richard Henderson <rth at redhat dot com>, Jason Merrill <jason at redhat dot com>, Diego Novillo <dnovillo at redhat dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
Date: Tue, 13 Aug 2002 23:50:21 -0700
Subject: Re: [tree-ssa] More CCP fixes [patch]

IMO the open PRs assigned to me related to bitfields, 6 or 7 of them,
show why having different types is a good idea.  GCC better understands
the range of non-overflow values, and gets sign promotion correct.
Having the same type makes promotion and various other things work
incorrectly, because of assumptions scattered around various parts
of GCC.

What goes wrong if we simply make explicit the truncations and extensions?

For example, consider:

 struct S {
   int i : 3;
 };

 S s;
 int j;

 s.i = j;
 j = s.i;

If the last two lines were represented as:

 s.i = j & 0x7;
 j = sign_extend(s.i);

would that work?

--
Mark Mitchell                mark@codesourcery.com
CodeSourcery, LLC            http://www.codesourcery.com

Follow-Ups:
- Re: [tree-ssa] More CCP fixes [patch]
  - From: Neil Booth
- Re: [tree-ssa] More CCP fixes [patch]
  - From: Diego Novillo

References:
- Re: [tree-ssa] More CCP fixes [patch]
  - From: Neil Booth

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