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Date: Sun, 19 Jul 2009 20:08:00 -0000
Message-ID: <af3ea28a0907191308v6579b003i99b050f511da146d@mail.gmail.com>
Subject: Re: how to use expand_builtin_alloca
From: ye janboe <janboe.ye@gmail.com>
To: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Ian Lance Taylor <iant@google.com>, Janboe Ye <yuan-bo.ye@motorola.com>, gcc@gcc.gnu.org
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Hi, Frank
>Those in turn
>might be represented with almost the normal mf_xform_decls(), while
>letting __builtin_alloca() remain.
How can we just remain __builtin_alloca() only for variable-length array?
Mudflap changes expand_builtin_alloca function.

I think it is enough for apply mudflap in Linux Kernel.

Janboe Ye


2009/7/16 Frank Ch. Eigler <fche@redhat.com>:
>
>> Janboe Ye <yuan-bo.ye@motorola.com> writes:
>
>>> normally gcc will use expand_builtin_alloca to handle variable array.
>>> But mudflap will force this function to return immediately to invoke
>>> alloca explicit.
>>>
>>> Is there some way to still use expand_builtin_alloca without changing
>>> gcc source code?
>
> I don't think so.
>
>
> Ian Lance Taylor <iant@google.com> writes:
>
>> mudflap can't check accesses to memory allocated using alloca unless
>> it overrides __builtin_alloca.
>
> It can't currently. =A0But instead of redirecting the call to a
> heap-based alloca() wannabe in libmudflap/mf-hooks1.c, perhaps
> mudflap could instrument alloca() by generating code like this
> instead:
>
> =A0__builtin_alloca(N) =A0--> =A0GIMPLE_TRY_FINALLY( try {
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0ptr =3D __=
builtin_alloca(N)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0__mf_regis=
ter(ptr ...)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0ptr;
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 } finally (attached t=
o the function scope) {
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0__mf_unreg=
ister(ptr ...)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
>
> Or perhaps not, if alloca() can be used in loops in way that
> prevents clean nesting of the try/finally.
>
> OTOH, I believe the original poster's case came from gcc-synthesized
> alloca's, coming from variable-length array allocation. =A0Those in turn
> might be represented with almost the normal mf_xform_decls(), while
> letting __builtin_alloca() remain.
>
> Either of these requires gcc changes though.
>
>
>> [...] =A0Although, of course, you could simply not use mudflap for the
>> code in question.
>
> The original poster's purpose is specifically to build bits of the
> linux kernel with mudflap instrumentation.
>
>
> - FChE
>