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Re: Replacing malloc with alloca.

From: Jeff Law <law at redhat dot com>
To: Florian Weimer <fw at deneb dot enyo dot de>, Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>
Cc: Richard Biener <richard dot guenther at gmail dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Vinod Kathail <vinodk at xilinx dot com>, Shail Aditya Gupta <shailadi at xilinx dot com>, Vidhumouli Hunsigida <vidhum at xilinx dot com>, Nagaraju Mekala <nmekala at xilinx dot com>
Date: Sun, 13 Sep 2015 12:56:40 -0600
Subject: Re: Replacing malloc with alloca.
Authentication-results: sourceware.org; auth=none
References: <37378DC5BCD0EE48BA4B082E0B55DFAA42990090 at XAP-PVEXMBX02 dot xlnx dot xilinx dot com> <87vbbenpga dot fsf at mid dot deneb dot enyo dot de>

On 09/13/2015 12:28 PM, Florian Weimer wrote:

* Ajit Kumar Agarwal:

The replacement of malloc with alloca can be done on the following
analysis.

If the lifetime of an object does not stretch beyond the immediate
scope. In such cases the malloc can be replaced with alloca.  This
increases the performance to a great extent.


You also need to make sure that the object is small (less than a page)
and that there is no deep recursion going on.  Otherwise, the program
may no longer work after the transformation with real-world restricted
stack sizes.  It may even end up with additional security issues.

You also have to make sure you're not inside a loop. Even a smallallocation inside a loop is problematical from a security standpoint.

You also need to look at what other objects might be on the stack andyou have to look at the functional scope, not the immediate scope asalloca space isn't returned until the end of a function.


jeff

Follow-Ups:
- Re: Replacing malloc with alloca.
  - From: Florian Weimer

References:
- Replacing malloc with alloca.
  - From: Ajit Kumar Agarwal
- Re: Replacing malloc with alloca.
  - From: Florian Weimer

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