Refactor Perl_av_fetch: remove goto, reduce the number of branches

Author: bbrtj

av.c

@@ -266,9 +266,6 @@ 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;
 
     if (UNLIKELY(SvRMAGICAL(av))) {
@@ -291,24 +288,23 @@ Perl_av_fetch(pTHX_ AV *av, SSize_t key, I32 lval)
         }
     }
 
-    neg  = (key < 0);
-    size = AvFILLp(av) + 1;
-    key += neg * size; /* handle negative index without using branch */
+    /* recalculating key early helps branch prediction */
+    SSize_t neg = key < 0;
+    SSize_t size = AvFILLp(av) + 1;
+    key += neg * size;
 
     /* 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;
-        goto emptiness;
-    }
-
-    if (!AvARRAY(av)[key]) {
-      emptiness:
-        return lval ? av_store(av,key,newSV_type(SVt_NULL)) : NULL;
-    }
-
-    return &AvARRAY(av)[key];
+    if ((Size_t)key >= (Size_t)size)
+        /* Only non-negative out of bounds keys are eligible for lvalue
+         * treatment, so adjust lval by setting value to 0 if the key was
+         * negative */
+        lval *= !neg;
+    else if (AvARRAY(av)[key])
+        return &AvARRAY(av)[key];
+
+    /* value does not exist in the array */
+    return lval ? av_store(av, key, newSV_type(SVt_NULL)) : NULL;
 }
 
 /*