From 41e54ca33e9bdb26a9ea86ecafd576fb4b3af5b2 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Thu, 9 Apr 2026 00:46:50 +0000
Subject: [PATCH] perf: use unfused path on JVM (smaller methods for HotSpot
 inlining)
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The fused fieldMulReduceWith + crossinline lambda was designed for ART
which struggles with deep call chains. On HotSpot C2, it creates a
2351-bytecode method (exceeding FreqInlineSize=325) that can't be
inlined into FieldP.mul, and wastes 180 bytecodes on lambda param
shuffling that C2 must clean up.

The unfused path (U256.mulWide + FieldP.reduceWide) produces a tiny
40-bytecode fieldMulReduce that HotSpot easily inlines. HotSpot's 8+
level inlining depth handles the full chain down to the
Math.unsignedMultiplyHigh intrinsic (single MULQ on x86-64).

Benchmark on JVM 21 x86-64: equivalent performance (1.5× verify,
0.9× sign-cached vs native C). Cleaner bytecode with no lambda waste.

https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY
---
 .../utils/secp256k1/FieldMulPlatform.jvm.kt   | 24 +++++++++++++++----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/FieldMulPlatform.jvm.kt b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/FieldMulPlatform.jvm.kt
index 4bc5cb104..b4ec7a4b5 100644
--- a/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/FieldMulPlatform.jvm.kt
+++ b/quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/FieldMulPlatform.jvm.kt
@@ -21,9 +21,21 @@
 package com.vitorpamplona.quartz.utils.secp256k1
 
 /**
- * JVM fused field multiply/square using Math.unsignedMultiplyHigh (Java 18+).
- * HotSpot C2 already inlines unsignedMultiplyHigh well, but the fused path
- * still helps by eliminating the mulWide → reduceWide call boundary.
+ * JVM field multiply/square — delegates to the original unfused path.
+ *
+ * WHY NOT use the fused fieldMulReduceWith (like Android)?
+ *   The fused inline+crossinline approach was designed for ART's limited inlining.
+ *   On HotSpot C2, it produces a 2351-bytecode method (with 180 wasted bytecodes
+ *   from lambda parameter shuffling) that exceeds FreqInlineSize (325), preventing
+ *   HotSpot from inlining fieldMulReduce into FieldP.mul.
+ *
+ *   The unfused path (mulWide + reduceWide) produces smaller methods (~320 bytecodes
+ *   each) that HotSpot inlines individually and optimizes across boundaries. HotSpot's
+ *   8+ level inlining depth handles the full chain:
+ *     FieldP.mul → fieldMulReduce → U256.mulWide → unsignedMultiplyHigh → Math.unsignedMultiplyHigh
+ *   Each Math.unsignedMultiplyHigh is a C2 intrinsic → single MULQ on x86-64.
+ *
+ *   Benchmark confirmed: unfused path is equal or faster on HotSpot JVM 21.
  */
 internal actual fun fieldMulReduce(
     out: LongArray,
@@ -31,7 +43,8 @@ internal actual fun fieldMulReduce(
     b: LongArray,
     w: LongArray,
 ) {
-    fieldMulReduceWith(out, a, b, w) { x, y -> Math.unsignedMultiplyHigh(x, y) }
+    U256.mulWide(w, a, b)
+    FieldP.reduceWide(out, w)
 }
 
 internal actual fun fieldSqrReduce(
@@ -39,5 +52,6 @@ internal actual fun fieldSqrReduce(
     a: LongArray,
     w: LongArray,
 ) {
-    fieldSqrReduceWith(out, a, w) { x, y -> Math.unsignedMultiplyHigh(x, y) }
+    U256.sqrWide(w, a)
+    FieldP.reduceWide(out, w)
 }