diff options
| author | naruse <naruse@b2dd03c8-39d4-4d8f-98ff-823fe69b080e> | 2017-02-02 15:54:51 +0000 |
|---|---|---|
| committer | naruse <naruse@b2dd03c8-39d4-4d8f-98ff-823fe69b080e> | 2017-02-02 15:54:51 +0000 |
| commit | 14b58d799bcc0c4457b6ff0010dcce2e4669a4fd (patch) | |
| tree | 1a1526b8206577a2b172836d875fc904d3b33f7f /configure.in | |
| parent | 9ae47fb45bcc531bcc289411a7a48281011176f1 (diff) | |
| download | ruby-14b58d799bcc0c4457b6ff0010dcce2e4669a4fd.tar.gz | |
Use carry flag to reduce instructions
NOTE:
(1) Fixnum's LSB is always 1.
It means you can always run `x - 1` without overflow.
(2) Of course `z = x + (y-1)` may overflow.
Now z's LSB is always 1, and the MSB of true result is also 1.
You can get true result in long as `(1<<63)|(z>>1)`,
and it equals to `(z<<63)|(z>>1)` == `ror(z)`.
GCC and Clang have __builtin_add_ovewflow:
* https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html
* https://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57506 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Diffstat (limited to 'configure.in')
| -rw-r--r-- | configure.in | 3 |
1 files changed, 3 insertions, 0 deletions
diff --git a/configure.in b/configure.in index 6558122797..05791864bb 100644 --- a/configure.in +++ b/configure.in @@ -2498,6 +2498,9 @@ RUBY_CHECK_BUILTIN_FUNC(__builtin_clzl, [__builtin_clzl(0)]) RUBY_CHECK_BUILTIN_FUNC(__builtin_clzll, [__builtin_clzll(0)]) RUBY_CHECK_BUILTIN_FUNC(__builtin_ctz, [__builtin_ctz(0)]) RUBY_CHECK_BUILTIN_FUNC(__builtin_ctzll, [__builtin_ctzll(0)]) +RUBY_CHECK_BUILTIN_FUNC(__builtin_add_ovewflow, [__builtin_add_ovewflow(0)]) +RUBY_CHECK_BUILTIN_FUNC(__builtin_sub_ovewflow, [__builtin_sub_ovewflow(0)]) +RUBY_CHECK_BUILTIN_FUNC(__builtin_mul_ovewflow, [__builtin_mul_ovewflow(0)]) RUBY_CHECK_BUILTIN_FUNC(__builtin_constant_p, [__builtin_constant_p(0)]) RUBY_CHECK_BUILTIN_FUNC(__builtin_choose_expr, [ [int x[__extension__(__builtin_choose_expr(1, 1, -1))]]; |