From 1c1cd8efc57884e3bae72f964584967645e232e3 Mon Sep 17 00:00:00 2001
From: Lance Pollard <lp@elk.fm>
Date: Sun, 1 Mar 2026 16:51:56 -0800
Subject: [PATCH] Make HEAP_CAP and MAX_THREADS configurable via compile-time
 defines

Wrap HEAP_CAP and MAX_THREADS in #ifndef guards so embedders can
tune them at compile time without modifying source.

Add a compile-time check that HEAP_CAP_BITS does not exceed VAL_BITS,
since heap addresses must fit in the 38-bit VAL field of a term.

Usage: clang -O2 -DHEAP_CAP_BITS=28 -DMAX_THREADS=4 -o main main.c

Defaults are unchanged. No behavior change on existing platforms.
---
 clang/hvm.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/clang/hvm.c b/clang/hvm.c
index ebf54fe..6d74bfc 100644
--- a/clang/hvm.c
+++ b/clang/hvm.c
@@ -155,10 +155,18 @@ typedef struct {
 // Capacities
 // ==========
 
-#define HEAP_CAP (1ULL << 38)
+#ifndef HEAP_CAP_BITS
+#define HEAP_CAP_BITS 38
+#endif
+#if HEAP_CAP_BITS > VAL_BITS
+#error "HEAP_CAP_BITS must be <= VAL_BITS (heap addresses must fit in term VAL field)"
+#endif
+#define HEAP_CAP (1ULL << HEAP_CAP_BITS)
 #define BOOK_CAP (1ULL << 24)
 #define WNF_CAP  (1ULL << 32)
+#ifndef MAX_THREADS
 #define MAX_THREADS 64
+#endif
 
 // Thread Globals
 // ==============