diff --git a/Zend/Optimizer/zend_inference.c b/Zend/Optimizer/zend_inference.c
index effff75e00f2..b5059ddf5400 100644
--- a/Zend/Optimizer/zend_inference.c
+++ b/Zend/Optimizer/zend_inference.c
@@ -760,12 +760,14 @@ static inline zend_long safe_shift_left(zend_long n, zend_long s) {
 }
 
 static inline bool shift_left_overflows(zend_long n, zend_long s) {
-	/* This considers shifts that shift in the sign bit to be overflowing as well */
-	if (n >= 0) {
-		return s >= SIZEOF_ZEND_LONG * 8 - 1 || safe_shift_left(n, s) < n;
-	} else {
-		return s >= SIZEOF_ZEND_LONG * 8 || safe_shift_left(n, s) > n;
+	if (UNEXPECTED((zend_ulong)s >= SIZEOF_ZEND_LONG * 8)) {
+		return true;
+	}
+	if (UNEXPECTED(s == SIZEOF_ZEND_LONG * 8 - 1)) {
+		return n != 0 && n != -1;
 	}
+	zend_long shifted = (zend_long)((zend_ulong)n << (zend_ulong)s);
+	return (shifted >> s) != n;
 }
 
 /* If b does not divide a exactly, return the two adjacent values between which the real result
diff --git a/Zend/zend_alloc.c b/Zend/zend_alloc.c
index b293ee1a51dc..61754ceb3c1a 100644
--- a/Zend/zend_alloc.c
+++ b/Zend/zend_alloc.c
@@ -1034,19 +1034,21 @@ static zend_always_inline void zend_mm_chunk_init(zend_mm_heap *heap, zend_mm_ch
 static zend_always_inline zend_mm_chunk *zend_mm_encode_cached_chunk(const zend_mm_heap *heap, const void *holder, const zend_mm_chunk *next)
 {
 #ifdef WORDS_BIGENDIAN
-	return (zend_mm_chunk*)((uintptr_t)next ^ heap->shadow_key ^ (uintptr_t)holder);
+	uintptr_t p = (uintptr_t)next;
 #else
-	return (zend_mm_chunk*)(BSWAPPTR((uintptr_t)next) ^ heap->shadow_key ^ (uintptr_t)holder);
+	uintptr_t p = BSWAPPTR((uintptr_t)next);
 #endif
+	/* ARX (Addition-XOR) non-linear mixing prevents linear algebraic shadow_key recovery */
+	return (zend_mm_chunk*)((p + heap->shadow_key) ^ (uintptr_t)holder);
 }
 
 static zend_always_inline zend_mm_chunk *zend_mm_decode_cached_chunk_key(uintptr_t key, const void *holder, const zend_mm_chunk *encoded)
 {
-#ifdef WORDS_BIGENDIAN
-	zend_mm_chunk *next = (zend_mm_chunk*)((uintptr_t)encoded ^ key ^ (uintptr_t)holder);
-#else
-	zend_mm_chunk *next = (zend_mm_chunk*)(BSWAPPTR((uintptr_t)encoded ^ key ^ (uintptr_t)holder));
+	uintptr_t p = ((uintptr_t)encoded ^ (uintptr_t)holder) - key;
+#ifndef WORDS_BIGENDIAN
+	p = BSWAPPTR(p);
 #endif
+	zend_mm_chunk *next = (zend_mm_chunk*)p;
 
 	ZEND_MM_CHECK(ZEND_MM_ALIGNED_OFFSET(next, ZEND_MM_CHUNK_SIZE) == 0, "zend_mm_heap corrupted");
 	return next;
diff --git a/Zend/zend_gc.c b/Zend/zend_gc.c
index cd7c3c512daf..9d31845efdc0 100644
--- a/Zend/zend_gc.c
+++ b/Zend/zend_gc.c
@@ -421,7 +421,10 @@ static zend_always_inline gc_root_buffer* gc_decompress(zend_refcounted *ref, ui
 
 	while (1) {
 		idx += GC_MAX_UNCOMPRESSED;
-		ZEND_ASSERT(idx < GC_G(first_unused));
+		if (UNEXPECTED(idx >= GC_G(first_unused))) {
+			zend_error_noreturn(E_CORE_ERROR, "GC root buffer corruption detected: unable to resolve compressed address");
+			return NULL;
+		}
 		root = GC_IDX2PTR(idx);
 		if (GC_GET_PTR(root->ref) == ref) {
 			return root;
diff --git a/main/streams/memory.c b/main/streams/memory.c
index c43ddca579b6..751b79948cb2 100644
--- a/main/streams/memory.c
+++ b/main/streams/memory.c
@@ -52,8 +52,12 @@ static ssize_t php_stream_memory_write(php_stream *stream, const char *buf, size
 	if (ms->mode & TEMP_STREAM_APPEND) {
 		ms->fpos = data_len;
 	}
-	if (ms->fpos + count > data_len) {
-		zend_string *grown = zend_string_frealloc(ms->data, ms->fpos + count, 0, /* fallible */ true);
+	if (UNEXPECTED(count > SIZE_MAX - ms->fpos)) {
+		return (ssize_t) -1;
+	}
+	size_t new_pos = ms->fpos + count;
+	if (new_pos > data_len) {
+		zend_string *grown = zend_string_frealloc(ms->data, new_pos, 0, /* fallible */ true);
 		if (UNEXPECTED(!grown)) {
 			return (ssize_t) -1;
 		}
@@ -69,7 +73,7 @@ static ssize_t php_stream_memory_write(php_stream *stream, const char *buf, size
 		ZEND_ASSERT(buf != NULL);
 		memcpy(ZSTR_VAL(ms->data) + ms->fpos, (char*) buf, count);
 		ZSTR_VAL(ms->data)[ZSTR_LEN(ms->data)] = '\0';
-		ms->fpos += count;
+		ms->fpos = new_pos;
 	}
 	return count;
 }
@@ -142,9 +146,13 @@ static int php_stream_memory_seek(php_stream *stream, zend_off_t offset, int whe
 					return 0;
 				}
 			} else {
+				if (UNEXPECTED((size_t)offset > SIZE_MAX - ms->fpos)) {
+					*newoffs = -1;
+					return -1;
+				}
 				stream->eof = 0;
 				stream->fatal_error = 0;
-				ms->fpos = ms->fpos + offset;
+				ms->fpos += (size_t)offset;
 				*newoffs = ms->fpos;
 				return 0;
 			}
@@ -154,7 +162,7 @@ static int php_stream_memory_seek(php_stream *stream, zend_off_t offset, int whe
 				*newoffs = -1;
 				return -1;
 			} else {
-				ms->fpos = offset;
+				ms->fpos = (size_t)offset;
 				*newoffs = ms->fpos;
 				stream->eof = 0;
 				stream->fatal_error = 0;
@@ -162,7 +170,11 @@ static int php_stream_memory_seek(php_stream *stream, zend_off_t offset, int whe
 			}
 		case SEEK_END:
 			if (offset > 0) {
-				ms->fpos = ZSTR_LEN(ms->data) + offset;
+				if (UNEXPECTED((size_t)offset > SIZE_MAX - ZSTR_LEN(ms->data))) {
+					*newoffs = -1;
+					return -1;
+				}
+				ms->fpos = ZSTR_LEN(ms->data) + (size_t)offset;
 				*newoffs = ms->fpos;
 				stream->eof = 0;
 				stream->fatal_error = 0;
diff --git a/main/streams/userspace.c b/main/streams/userspace.c
index 8c4b43f6ce83..ec18f3dda699 100644
--- a/main/streams/userspace.c
+++ b/main/streams/userspace.c
@@ -612,6 +612,9 @@ static ssize_t php_userstreamop_write(php_stream *stream, const char *buf, size_
 	} else {
 		convert_to_long(&retval);
 		didwrite = Z_LVAL(retval);
+		if (didwrite < 0) {
+			didwrite = -1;
+		}
 	}
 
 	/* don't allow strange buffer overruns due to bogus return */