Fix GCC 4.7 warnings.

Fix GCC 4.7 warnings.

* src/string-search.h (StringSearch): Avoid -Werror=strict-overflow
  warning.

Committed: http://code.google.com/p/v8/source/detail?r=10295

[Tobias Burnus, 2011-12-15 17:19:05]

RFC patch to fix the remaining GCC 4.7 warnings, which prevent building due to
-Werror.

a) -Werror=unused-but-set-variable
* src/ia32/lithium-codegen-ia32.cc: "address" is set but not used.
* src/x64/lithium-codegen-x64.cc: "object" is set but only used in an ASSERT,
which only expands to something when DEBUG is defined.


b) src/string-search.h: -Werror=strict-overflow
This one is a bit more complex. One gets the error:

src/string-search.h:404:3: error: assuming signed overflow does not occur when
assuming that (X + c) < X is always false [-Werror=strict-overflow]

The code in question looks as follows:

  int suffix = pattern_length + 1;
     int i = pattern_length;
     while (i > start) {
        // modifies suffix
     }
  if (suffix == pattern_length) {
  }

The compiler now does jump threading by adding an "if (pattern_length <= start)"
and jumps over both the "while" and the "if". However, that's only valid if
"pattern_length + 1" does not overflow.

To silence the compiler, one can add a simple:
  if (pattern_length <= start)
    return;

However, one could also define the following (cf. for instance the Linux kernel)
and use it:

#ifndef __GNUC__
#define likely(x)       (x)
#define unlikely(x)     (x)
#else
#define likely(x)       __builtin_expect(!!(x), 1)
#define unlikely(x)     __builtin_expect(!!(x), 0)
#endif

if (unlikely(pattern_length <= start))
  return;

which should be a bit quicker. (I assume that "pattern_length > start" is the
common case. Though, I have not really thought about it. If my assumption is
wrong, a normal "if" is better.)

[Tobias Burnus, 2011-12-20 14:42:19]

Updated patch - the changes of Rev. 10280 made the change to
src/{ia32,x64}/lithium-codegen-{ia32,x86}.cc obsolete.

Thus, only src/string-search.h remains as RFC patch.

(Together with the patch of issue 8971002, the current v8 trunk with the today's
GCC 4.7 version compiles without any error/warning.)

[Vyacheslav Egorov (Chromium), 2011-12-20 14:58:54]

http://codereview.chromium.org/8947022/diff/3001/src/string-search.h
File src/string-search.h (right):

http://codereview.chromium.org/8947022/diff/3001/src/string-search.h#newcode372
src/string-search.h:372: if (pattern_length <= start)
we either put return on the same line as if or we use curly-brackets.

[Tobias Burnus, 2011-12-20 15:26:28]

Fixed indention style. (Sorry for accidentally using GNU style.)

[Vyacheslav Egorov (Chromium), 2011-12-20 15:30:10]

lgtm

[Vyacheslav Egorov (Chromium), 2011-12-20 15:30:18]

I will land.