From: Xi Ruoyao <xry111@mengyan1223.wang>
To: "relliott@umn.edu" <relliott@umn.edu>
Cc: gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: High memory usage compiling large ‘xxd -i’ output
Date: Sun, 12 Apr 2020 10:07:52 +0800 [thread overview]
Message-ID: <149f3853f0a495a5f993d38ad60567fa6d10a745.camel@mengyan1223.wang> (raw)
In-Reply-To: <3F1B5DA6-7224-4431-8070-123D18B69B3C@umn.edu>
On 2020-04-11 16:31 -0500, relliott--- via Gcc-help wrote:
> Hello,
>
> Thanks for your reply. I guess I don’t understand why this requires so much
> memory all at once. Is it not possible to write to the object file as the
> initializer is parsed?
A compiler doesn't write to object file. It only writes to assembly file.
Modern compilers are designed multi-layer so the parser (frontend) should not
output assembly (it's the job of the backend). And, doing that so will miss
optimizations. A simple code:
const int b[] = {0, 1, 2};
int foo(void)
{
int i, x = 0;
for (i = 0; i < 3; i++)
x += b[i];
return x;
}
The body of "foo" will be optimized to "return 3;" at -O2. If we wrote the
array content to assembly file and discarded the value in memory, this
optimization will be impossible.
"xxd" a binary file and then compile the result with a compiler is just like
"converting a .jpg to .png then back to .jpg". A smarter way:
ld -r -b binary some_big_binary_blob.bin -o some_big_binary_blob.o
There will be symbols _binary_some_big_binary_blob_bin_start,
_binary_some_big_binary_blob_end in some_big_binary_blob.o. Then it's possible
to access the content of the binary blob with:
extern const char _binary_some_big_binary_blob_bin_start;
extern const char _binary_some_big_binary_blob_bin_end;
int main(void)
{
const char *begin = &_binary_some_big_binary_blob_bin_start;
const char *end = &_binary_some_big_binary_blob_bin_end;
const char *p;
for (p = begin; p != end; p++)
play_with(*p);
}
--
Xi Ruoyao <xry111@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University
next prev parent reply other threads:[~2020-04-12 2:08 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-11 20:03 relliott
2020-04-11 21:22 ` Jonathan Wakely
2020-04-11 21:31 ` relliott
2020-04-12 2:07 ` Xi Ruoyao [this message]
2020-04-12 22:01 ` relliott
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