Optimise IntToString by avoiding string allocation.

Optimise IntToString by avoiding string allocation.

Use a fixed-sized temporary buffer on the stack for IntToString and
variants, rather than allocating a string.

This reduces the time to run IntToString in a microbenchmark by 70%, and
the code size by 30%, both on Android arm7. See the bug for more details.

Since all the *Int*ToString* functions share a common implementation,
they all benefit from this change.

Security implications: If this function overflowed its buffer, then this
change would change a heap overflow bug to a stack overflow
bug. However, the unit tests in combination with the DCHECKs in the code
provide a strong guarantee that this function does not overflow its
buffer. The compiler also adds stack-smashing protection.

BUG=535972
TEST=base_unittests

Committed: https://crrev.com/ad547d8741b466f19d50adbc0abb2b6a4b5524e8
Cr-Commit-Position: refs/heads/master@{#350883}

[Adam Rice, 2015-09-25 15:31:04]

ricea@chromium.org changed reviewers:
+ mark@chromium.org

[Mark Mentovai, 2015-09-25 15:35:28]

LGTM otherwise

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
File base/strings/string_number_conversions.cc (right):

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
base/strings/string_number_conversions.cc:35: typedef typename STR::value_type
CHR;
https://chromium-cpp.appspot.com/: use “using CHR = typename STR::value_type;”
instead.

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
base/strings/string_number_conversions.cc:42: CHR* i = outbuf + kOutputBufSize;
You can save outbuf + kOutputBufSize in an “end” variable so that you don’t have
to compute the same thing again when you create the return STR below.

[Adam Rice, 2015-09-25 17:35:36]

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
File base/strings/string_number_conversions.cc (right):

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
base/strings/string_number_conversions.cc:35: typedef typename STR::value_type
CHR;
On 2015/09/25 at 15:35:28, Mark Mentovai - August is over wrote:
> https://chromium-cpp.appspot.com/: use “using CHR = typename STR::value_type;”
instead.

Done.

https://codereview.chromium.org/1359253003/diff/1/base/strings/string_number_...
base/strings/string_number_conversions.cc:42: CHR* i = outbuf + kOutputBufSize;
On 2015/09/25 at 15:35:28, Mark Mentovai - August is over wrote:
> You can save outbuf + kOutputBufSize in an “end” variable so that you don’t
have to compute the same thing again when you create the return STR below.

Done.

[Mark Mentovai, 2015-09-25 17:37:09]

LGTM

[Adam Rice, 2015-09-25 18:36:35]

The CQ bit was checked by ricea@chromium.org

[commit-bot: I haz the power, 2015-09-25 18:37:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1359253003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1359253003/20001

[commit-bot: I haz the power, 2015-09-25 19:27:58]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-25 19:28:43]

Patchset 2 (id:??) landed as
https://crrev.com/ad547d8741b466f19d50adbc0abb2b6a4b5524e8
Cr-Commit-Position: refs/heads/master@{#350883}