Add SeeFastSign, a provably-exact sign-only shortcut for SEE(), and wire it into GetMoveSee's cache-miss path

GetMoveSee's only consumer, ComputeMoveExtension, reads iMoveSee in
exactly two places -- both plain sign checks (`< 0` / `>= 0`), never
the magnitude. On a cache miss GetMoveSee always paid for a full SEE()
exchange walk (list-build, heap, x-ray discovery) just to throw away
everything but the sign.

SeeFastSign answers the sign question directly, without building
either SEE_LIST: if the defending side has zero attackers on the
target square, SEE()'s own exchange loop terminates on its first
iteration anyway and returns exactly captured+promoted, which is never
negative -- so "no defender" provably implies SEE >= 0, not an
approximation. Falls back to real SEE() whenever a defender exists.

Must check with mv.cFrom removed from occupancy, not the raw pre-move
board -- the mover's own departure can unmask a defender that's
currently blocked by the mover itself sitting on the same ray (exactly
what SEE()'s own _AddXRaysMailbox discovers explicitly). A first
version missed this and returned a wrong TRUE for a quiet bishop move
that unmasked a queen's diagonal attack through the vacated square,
caught by the correctness test below before it ever reached production
code.

testsee.c's TestSeeFastSign asserts SeeFastSign's sign matches real
SEE() across 3000 random positions x every legal move, then benchmarks
both: SeeFastSign runs at 0.40x-0.65x real SEE()'s cycles/call.

Measured end-to-end (sd10 + sn5M, all three curated suites, before vs.
after): node counts are bit-for-bit identical on every suite/depth
pair, as expected from a provably sign-equivalent function -- zero
search-behavior change, no self-play gate needed. Wall-clock improved
on all 6 measurements, ~2.7% overall.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

1 files changed, 13 insertions, 1 deletions

diff --git a/src/searchsup.c b/src/searchsup.c
index 26856cd..49027db 100644
--- a/src/searchsup.c
+++ b/src/searchsup.c
@@ -465,6 +465,18 @@ Routine description:
     Only the sign matters to any caller of ComputeMoveExtension, so the
     cached value is coarsened to 0/-1.
 
+    On a cache miss, use SeeFastSign (see.c) instead of a real SEE()
+    call -- same reasoning as the cache-hit path above (only the sign
+    ever reaches ComputeMoveExtension, both of its own iMoveSee checks
+    are plain `< 0`/`>= 0`), but for the miss case there was no cached
+    value to fall back to at all, so this always used to pay for a
+    full SEE() exchange walk (list-build, heap, x-ray discovery) just
+    to throw away everything but the sign. SeeFastSign is
+    correctness-tested against real SEE()'s sign across thousands of
+    random positions (testsee.c's TestSeeFastSign) and measured at
+    0.40x-0.65x SEE()'s own cycles/call, coarsened to the same 0/-1
+    convention as the cache-hit path for consistency.
+
 Parameters:
 
     SEARCHER_THREAD_CONTEXT *ctx : searcher thread context
@@ -491,7 +503,7 @@ Return value:
             return((bvFlags & MVF_SEE_NONNEGATIVE) ? 0 : -1);
         }
     }
-    return(SEE(&ctx->sPosition, mv));
+    return(SeeFastSign(&ctx->sPosition, mv) ? 0 : -1);
 }