diff --git a/include/jemalloc/internal/sc.h b/include/jemalloc/internal/sc.h
index 17a8278a..7d2ff271 100644
--- a/include/jemalloc/internal/sc.h
+++ b/include/jemalloc/internal/sc.h
@@ -268,12 +268,17 @@
 #define SC_LARGE_MINCLASS ((size_t)1ULL << (LG_PAGE + SC_LG_NGROUP))
 #define SC_LG_LARGE_MINCLASS (LG_PAGE + SC_LG_NGROUP)
 
-/* Internal; only used for the definition of SC_LARGE_MAXCLASS. */
-#define SC_MAX_BASE ((size_t)1 << (SC_PTR_BITS - 2))
-#define SC_MAX_DELTA ((size_t)1 << (SC_PTR_BITS - 2 - SC_LG_NGROUP))
-
-/* The largest size class supported. */
-#define SC_LARGE_MAXCLASS (SC_MAX_BASE + (SC_NGROUP - 1) * SC_MAX_DELTA)
+/*
+ * The largest size class supported.  Spell this out directly to avoid
+ * expanding subtractive arithmetic at every use site.
+ */
+#if LG_SIZEOF_PTR == 3
+#	define SC_LARGE_MAXCLASS 0x7000000000000000ULL
+#elif LG_SIZEOF_PTR == 2
+#	define SC_LARGE_MAXCLASS 0x70000000ULL
+#else
+#	error "Unsupported pointer size"
+#endif
 
 /* Maximum number of regions in one slab. */
 #ifndef CONFIG_LG_SLAB_MAXREGS
diff --git a/include/jemalloc/internal/sz.h b/include/jemalloc/internal/sz.h
index d75a3034..3ee8a6b3 100644
--- a/include/jemalloc/internal/sz.h
+++ b/include/jemalloc/internal/sz.h
@@ -314,12 +314,20 @@ sz_size2index_usize_fastpath(size_t size, szind_t *ind, size_t *usize) {
 
 JEMALLOC_ALWAYS_INLINE size_t
 sz_s2u_compute_using_delta(size_t size) {
+	if (unlikely(size > SC_LARGE_MAXCLASS)) {
+		return 0;
+	}
+
 	size_t x = lg_floor((size << 1) - 1);
 	size_t lg_delta = (x < SC_LG_NGROUP + LG_QUANTUM + 1)
 	    ? LG_QUANTUM
 	    : x - SC_LG_NGROUP - 1;
 	size_t delta = ZU(1) << lg_delta;
 	size_t delta_mask = delta - 1;
+	if (unlikely(size > SIZE_T_MAX - delta_mask)) {
+		return 0;
+	}
+
 	size_t usize = (size + delta_mask) & ~delta_mask;
 	return usize;
 }
@@ -385,7 +393,11 @@ JEMALLOC_ALWAYS_INLINE size_t
 sz_sa2u(size_t size, size_t alignment) {
 	size_t usize;
 
-	assert(alignment != 0 && ((alignment - 1) & alignment) == 0);
+	if (unlikely(alignment == 0)) {
+		return 0;
+	}
+	size_t alignment_mask = alignment - 1;
+	assert((alignment_mask & alignment) == 0);
 
 	/* Try for a small size class. */
 	if (size <= SC_SMALL_MAXCLASS && alignment <= PAGE) {
@@ -403,7 +415,10 @@ sz_sa2u(size_t size, size_t alignment) {
 		 *    144 | 10100000 |  32
 		 *    192 | 11000000 |  64
 		 */
-		usize = sz_s2u(ALIGNMENT_CEILING(size, alignment));
+		if (unlikely(size > SIZE_T_MAX - alignment_mask)) {
+			return 0;
+		}
+		usize = sz_s2u((size + alignment_mask) & ~alignment_mask);
 		if (usize < SC_LARGE_MINCLASS) {
 			return usize;
 		}
diff --git a/test/unit/size_classes.c b/test/unit/size_classes.c
index 5379047c..27f8cefd 100644
--- a/test/unit/size_classes.c
+++ b/test/unit/size_classes.c
@@ -190,6 +190,9 @@ TEST_BEGIN(test_overflow) {
 	max_size_class = get_max_size_class();
 	max_psz = max_size_class + PAGE;
 
+	expect_zu_eq(max_size_class, SC_LARGE_MAXCLASS,
+	    "Computed max size class should match SC_LARGE_MAXCLASS");
+
 	expect_u_eq(sz_size2index(max_size_class + 1), SC_NSIZES,
 	    "sz_size2index() should return NSIZES on overflow");
 	expect_u_eq(sz_size2index(ZU(PTRDIFF_MAX) + 1), SC_NSIZES,
@@ -203,6 +206,14 @@ TEST_BEGIN(test_overflow) {
 	    "sz_s2u() should return 0 for unsupported size");
 	expect_zu_eq(
 	    sz_s2u(SIZE_T_MAX), 0, "sz_s2u() should return 0 on overflow");
+	expect_zu_eq(sz_s2u_compute_using_delta(SC_LARGE_MAXCLASS + 1), 0,
+	    "sz_s2u_compute_using_delta() should return 0 for unsupported size");
+	expect_zu_eq(sz_s2u_compute_using_delta(SIZE_T_MAX), 0,
+	    "sz_s2u_compute_using_delta() should return 0 on overflow");
+	expect_zu_eq(sz_sa2u(1, 0), 0,
+	    "sz_sa2u() should return 0 for zero alignment");
+	expect_zu_eq(sz_sa2u(SIZE_T_MAX, PAGE), 0,
+	    "sz_sa2u() should return 0 on overflow");
 
 	expect_u_eq(sz_psz2ind(max_size_class + 1), SC_NPSIZES,
 	    "sz_psz2ind() should return NPSIZES on overflow");