From f4d8be8b395cdacf05f8c461fb7e9ce0366ff56f Mon Sep 17 00:00:00 2001 From: David Mitchell Date: Wed, 17 Aug 2016 12:06:27 +0100 Subject: av_fetch(): use less branches. The code that handles negative array indexes and out-of-bounds negative indices used to require: 2 conditions for a +ve index 3 conditions for a -ve index After this commit, for the common case where the index is in bounds, it requires a single condition regardless of sign. For the less common case of out-of-bounds, it requires 2 conditions. Also, the one condition is more branch-predict friendly - it's whether the index is in bounds or not. Previously the first test was whether key < 0, and in code that does mixed signs, such as $a[0] + $a[-1], branch prediction could be tricky. It achieves this at the expense of a more complex expression for the key. --- av.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) (limited to 'av.c') diff --git a/av.c b/av.c index 549ca6c86e..8f8cda5c57 100644 --- a/av.c +++ b/av.c @@ -244,6 +244,9 @@ S_adjust_index(pTHX_ AV *av, const MAGIC *mg, SSize_t *keyp) SV** Perl_av_fetch(pTHX_ AV *av, SSize_t key, I32 lval) { + SSize_t neg; + SSize_t size; + PERL_ARGS_ASSERT_AV_FETCH; assert(SvTYPE(av) == SVt_PVAV); @@ -268,15 +271,19 @@ Perl_av_fetch(pTHX_ AV *av, SSize_t key, I32 lval) } } - if (key < 0) { - key += AvFILLp(av) + 1; - if (UNLIKELY(key < 0)) + neg = (key < 0); + size = AvFILLp(av) + 1; + key += neg * size; /* handle negative index without using branch */ + + /* the cast from SSize_t to Size_t allows both (key < 0) and (key >= size) + * to be tested as a single condition */ + if ((Size_t)key >= (Size_t)size) { + if (UNLIKELY(neg)) return NULL; - assert(key <= AvFILLp(av)); - if (!AvARRAY(av)[key]) - goto emptyness; + goto emptyness; } - else if (key > AvFILLp(av) || !AvARRAY(av)[key]) { + + if (!AvARRAY(av)[key]) { emptyness: return lval ? av_store(av,key,newSV(0)) : NULL; } -- cgit v1.2.1