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Re: -fno-align-functions still adds padding on i386 on gcc3.1



On 17-May-2002 Richard Henderson wrote:
> See PR 6627, and the attached analysis.

Thanks.  Someone else discovered that the additional function padding
wasn't actually making up that much of a difference but that using
-fno-guess-branch-probability cut down on the generated code size
considerably.  Perhaps -Os should not enable this optimization?

An extract from the commit message in which -fno-guess-branch-probability
was turned on is here:

Saved 176 bytes by compiling with -fno-guess-branch-probability.  The
default of -fguess-branch-probablility causes time optimizations (?)
like rewriting `if (foo) x++;' as
`if (!foo) goto forth; back: ; ...; forth: x++; goto back;".  This is
pessimizes space especially well on i386's because one short branch
gets converted to 2 long ones.

If there is interest I can try to produce a small test case to demonstrate
that this optimization increases code size.

-- 

John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/


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