From 038ccbd112d26d15b7433b86bf48b1be0fea6e2f Mon Sep 17 00:00:00 2001
From: naruse <naruse@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Thu, 2 Feb 2017 15:54:51 +0000
Subject: 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
---
 internal.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

(limited to 'internal.h')

diff --git a/internal.h b/internal.h
index 3e5550f4ca..4ed1e41440 100644
--- a/internal.h
+++ b/internal.h
@@ -348,6 +348,53 @@ VALUE rb_int128t2big(int128_t n);
 
 #define ST2FIX(h) LONG2FIX((long)(h))
 
+static inline unsigned long
+rb_ulong_rotate_right(unsigned long x)
+{
+    return (x >> 1) | (x << (SIZEOF_LONG * CHAR_BIT - 1));
+}
+
+static inline VALUE
+rb_fix_plus_fix(VALUE x, VALUE y)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_ADD_OVERFLOW
+    long lz;
+    /* 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)`.
+     */
+    if (__builtin_add_overflow((long)x, (long)y-1, &lz)) {
+	return rb_int2big(rb_ulong_rotate_right((unsigned long)lz));
+    }
+    else {
+	return (VALUE)lz;
+    }
+#else
+    long lz = FIX2LONG(x) + FIX2LONG(y);
+    return LONG2NUM(lz);
+#endif
+}
+
+static inline VALUE
+rb_fix_minus_fix(VALUE x, VALUE y)
+{
+#ifdef HAVE_BUILTIN___BUILTIN_SUB_OVERFLOW
+    long lz;
+    if (__builtin_sub_overflow((long)x, (long)y-1, &lz)) {
+	return rb_int2big(rb_ulong_rotate_right((unsigned long)lz));
+    }
+    else {
+	return (VALUE)lz;
+    }
+#else
+    long lz = FIX2LONG(x) - FIX2LONG(y);
+    return LONG2NUM(lz);
+#endif
+}
 
 /* arguments must be Fixnum */
 static inline VALUE
-- 
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