Re: GIMPLE temporary variables

9 msg	Mercurial repository set up for GCC that mirror...

GIMPLE temporary variables
\ Andrea Callia D'Iddio (8 May 2007)
. \ Diego Novillo (8 May 2007)
. \ J.C. Pizarro (8 May 2007)
. . \ Joe Buck (8 May 2007)

2 msg	Improper Frame Description Entry - DWARF2 debug...
4 msg	PR31264 on 4.2.0 branch
9 msg	MinGW, GCC Vista,
3 msg	tuples: call for help
4 msg	One more df-branch benchmarking on SPEC2000
13 msg	Combined tree builds for mingw32
17 msg	libffi & powerpc
16 msg	live insns deleted by delete_trivially_dead_ins...
2 msg	Getting the name of the file to compile
2 msg	[cxx0x-branch] Rvalue references vs RVO
1 msg	Missing features on Changes for GCC 4.3
6 msg	auto vectorization - should this work ?
10 msg	Successful build of GCC 4.2.0 RC3 on latest Cyg...
2 msg	including PR summary in gcc-cvs commit mails
12 msg	libjava Divide_1 and pr6388 fail on 4.2.0 RC3 f...
3 msg	Regression in gfortran.fortran-torture/execute/...
1 msg	gcc-4.3-20070504 is now available
1 msg	Build status for GCC 3.4.5

Subject:	Re: GIMPLE temporary variables
Group:	Gcc
From:	Joe Buck
Date:	8 May 2007

On Tue, May 08, 2007 at 09:43:58PM +0200, J.C. Pizarro wrote:
> Andrea Callia D'Iddio <andrea.calliadiddio> wrote:
> >Hi all,
> >
> >I'm writing a new compilation pass in gcc, and I'm working on GIMPLE
> >code. When gcc produce GIMPLE code, it creates new temporary
> >variables, in order to simplify expressions and statements. For
> >example, if source C file contains
> >a=a+b+19;
> >then GIMPLE code is
> >D.1295 = a + b;
> >a = D.1295 + 19;
> >how can I recognize temporary variables, such as D.1295?
> >Thanks for support! bye!!
> >
> >
> >Andrea Callia D'Iddio
>
> This GIMPLE code is bad.
>
> It's better without a new temporary variable D.xxxx:
> a = a + b;
> a = a + 19;

No, it isn't; since you don't understand what the goal is, your notion of
what is good or bad is seriously confused.

Your proposed rewrite hides the distinction between generated tempories
and assignments written by the user (probably making -O0 code worse, if
the compiler thinks every write to a has to be flushed to memory), and
your idea that avoiding introducing temporaries is good ignores the fact
that the gimple code is fed to an SSA pass, where every distinct
assignment will create a new symbol.