[PATCH v7] eal: RTE_PTR_ADD/SUB char* for compiler optimizations

scott.k.mitch1
2026-01-14T21:58:10Z

From: Scott Mitchell <scott.k.mitch1@gmail.com>

Modify RTE_PTR_ADD and RTE_PTR_SUB to use char* pointer arithmetic
on Clang instead of uintptr_t casts. Benefits of this approach:
- API compatibility: works for both integer and pointer inputs
- Retains simple macros: no pragmas, no _Generic
- Enables Clang optimizations: Clang can now unroll and vectorize
pointer loops. GCC uses uintptr_t to avoid false positive warnings.

Example use case which benefits is __rte_raw_cksum. Performance
results from cksum_perf_autotest on Intel Xeon (Cascade Lake,
AVX-512) built with Clang 18.1 (TSC cycles/byte):
Block size Before After Improvement
100 0.40 0.24 ~40%
1500 0.50 0.06 ~8x
9000 0.49 0.06 ~8x

Signed-off-by: Scott Mitchell <scott.k.mitch1@gmail.com>

---
v7:
- Fix tests: use TEST_BUFFER_SIZE macro for buffer allocation
- Fix tests: ADD then SUB same amount to avoid out-of-bounds pointer arithmetic
- All RTE_PTR_SUB tests now verify round-trip (ADD+SUB returns to original)

v6:
- Make char* optimization Clang-only to avoid GCC false positive warnings
- Improve tests: use named constants instead of magic numbers
- Improve tests: use void* casts on expected values instead of uintptr_t on results

v5:
- Initial implementation with char* arithmetic for all compilers

v4:
- Used _Generic for type-based dispatch with char* casts
- Had warnings on both Clang and GCC due to _Generic type-checking all branches
---
app/test/meson.build | 1 +
app/test/test_ptr_add_sub.c | 197 +++++++++++++++++++++++++++++++++++
lib/eal/include/rte_common.h | 14 ++-
3 files changed, 208 insertions(+), 4 deletions(-)
create mode 100644 app/test/test_ptr_add_sub.c

diff --git a/app/test/meson.build b/app/test/meson.build
index efec42a6bf..80a19d65ba 100644
--- a/app/test/meson.build
+++ b/app/test/meson.build
@@ -152,6 +152,7 @@ source_file_deps = {
'test_power_intel_uncore.c': ['power', 'power_intel_uncore'],
'test_power_kvm_vm.c': ['power', 'power_kvm_vm'],
'test_prefetch.c': [],
+ 'test_ptr_add_sub.c': [],
'test_ptr_compress.c': ['ptr_compress'],
'test_rand_perf.c': [],
'test_rawdev.c': ['rawdev', 'bus_vdev', 'raw_skeleton'],
diff --git a/app/test/test_ptr_add_sub.c b/app/test/test_ptr_add_sub.c
new file mode 100644
index 0000000000..2956eabeb3
--- /dev/null
+++ b/app/test/test_ptr_add_sub.c
@@ -0,0 +1,197 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright(c) 2026 Apple Inc.
+ */
+
+#include "test.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#include <rte_common.h>
+
+/* Test constants for large integer types (16-bit and wider) */
+#define TEST_INITVAL_LARGE 0x1000
+#define TEST_INCREMENT_LARGE 100
+#define TEST_RETVAL_LARGE ((void *)0x1064)
+#define TEST_SUBVAL_LARGE ((void *)(TEST_INITVAL_LARGE - TEST_INCREMENT_LARGE))
+
+/* Test constants for 8-bit types */
+#define TEST_INITVAL_SMALL 100
+#define TEST_INCREMENT_SMALL 50
+#define TEST_RETVAL_SMALL ((void *)150)
+#define TEST_SUBVAL_SMALL ((void *)50)
+
+/* Test constants for bool (1-bit) */
+#define TEST_INITVAL_BOOL 1
+#define TEST_INCREMENT_BOOL 99
+#define TEST_RETVAL_BOOL ((void *)100)
+
+/* Buffer size for pointer tests */
+#define TEST_BUFFER_SIZE (TEST_INCREMENT_LARGE + TEST_INCREMENT_SMALL + 256)
+
+/* Test all C11 standard integer types */
+static int
+test_ptr_add_sub_integer_types(void)
+{
+ unsigned long long ull = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(ull, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for unsigned long long");
+ TEST_ASSERT_EQUAL(RTE_PTR_SUB(ull, TEST_INCREMENT_LARGE), TEST_SUBVAL_LARGE,
+ "RTE_PTR_SUB failed for unsigned long long");
+
+ long long ll = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(ll, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for long long");
+
+ unsigned long ul = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(ul, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for unsigned long");
+
+ long l = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(l, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for long");
+
+ unsigned int ui = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(ui, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for unsigned int");
+
+ int i = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(i, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for int");
+
+ unsigned short us = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(us, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for unsigned short");
+
+ short s = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(s, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for short");
+
+ unsigned char uc = TEST_INITVAL_SMALL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(uc, TEST_INCREMENT_SMALL), TEST_RETVAL_SMALL,
+ "RTE_PTR_ADD failed for unsigned char");
+
+ signed char sc = TEST_INITVAL_SMALL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(sc, TEST_INCREMENT_SMALL), TEST_RETVAL_SMALL,
+ "RTE_PTR_ADD failed for signed char");
+
+ char c = TEST_INITVAL_SMALL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(c, TEST_INCREMENT_SMALL), TEST_RETVAL_SMALL,
+ "RTE_PTR_ADD failed for char");
+
+ _Bool b = TEST_INITVAL_BOOL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(b, TEST_INCREMENT_BOOL), TEST_RETVAL_BOOL,
+ "RTE_PTR_ADD failed for _Bool");
+
+ bool b2 = TEST_INITVAL_BOOL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(b2, TEST_INCREMENT_BOOL), TEST_RETVAL_BOOL,
+ "RTE_PTR_ADD failed for bool");
+
+ return 0;
+}
+
+/* Test pointer types including const correctness */
+static int
+test_ptr_add_sub_pointer_types(void)
+{
+ char buffer[TEST_BUFFER_SIZE];
+ void *result;
+
+ /* Test void* and const void* */
+ void *vp = buffer;
+ result = RTE_PTR_ADD(vp, TEST_INCREMENT_LARGE);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_LARGE);
+ TEST_ASSERT_EQUAL(result, vp, "RTE_PTR_ADD/SUB failed for void*");
+
+ const void *cvp = buffer;
+ result = RTE_PTR_ADD(cvp, TEST_INCREMENT_SMALL);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_SMALL);
+ TEST_ASSERT_EQUAL(result, (const void *)cvp,
+ "RTE_PTR_ADD/SUB failed for const void*");
+
+ /* Test char* and const char* */
+ char *cp = buffer;
+ result = RTE_PTR_ADD(cp, TEST_INCREMENT_LARGE);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_LARGE);
+ TEST_ASSERT_EQUAL(result, (void *)cp, "RTE_PTR_ADD/SUB failed for char*");
+
+ const char *ccp = buffer;
+ result = RTE_PTR_ADD(ccp, TEST_INCREMENT_SMALL);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_SMALL);
+ TEST_ASSERT_EQUAL(result, (const void *)ccp,
+ "RTE_PTR_ADD/SUB failed for const char*");
+
+ /* Test uint32_t* and const uint32_t* */
+ uint32_t *u32p = (uint32_t *)buffer;
+ result = RTE_PTR_ADD(u32p, TEST_INCREMENT_LARGE);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_LARGE);
+ TEST_ASSERT_EQUAL(result, (void *)u32p,
+ "RTE_PTR_ADD/SUB failed for uint32_t*");
+
+ const uint32_t *cu32p = (const uint32_t *)buffer;
+ result = RTE_PTR_ADD(cu32p, TEST_INCREMENT_SMALL);
+ result = RTE_PTR_SUB(result, TEST_INCREMENT_SMALL);
+ TEST_ASSERT_EQUAL(result, (const void *)cu32p,
+ "RTE_PTR_ADD/SUB failed for const uint32_t*");
+
+ /* Verify assigning to const pointer works (adding const is safe) */
+ const void *result_const = RTE_PTR_ADD(cvp, TEST_INCREMENT_LARGE);
+ result_const = RTE_PTR_SUB(result_const, TEST_INCREMENT_LARGE);
+ TEST_ASSERT_EQUAL(result_const, cvp,
+ "RTE_PTR_ADD/SUB failed when assigning to const void*");
+
+ return 0;
+}
+
+/* Test that typedefs resolve to native types correctly */
+static int
+test_ptr_add_sub_typedefs(void)
+{
+ uint64_t u64 = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(u64, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for uint64_t");
+
+ uint32_t u32 = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(u32, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for uint32_t");
+
+ uint16_t u16 = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(u16, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for uint16_t");
+
+ uint8_t u8 = TEST_INITVAL_SMALL;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(u8, TEST_INCREMENT_SMALL), TEST_RETVAL_SMALL,
+ "RTE_PTR_ADD failed for uint8_t");
+
+ uintptr_t uptr = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(uptr, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for uintptr_t");
+
+ size_t sz = TEST_INITVAL_LARGE;
+ TEST_ASSERT_EQUAL(RTE_PTR_ADD(sz, TEST_INCREMENT_LARGE), TEST_RETVAL_LARGE,
+ "RTE_PTR_ADD failed for size_t");
+
+ return 0;
+}
+
+/* Main test function that runs all subtests */
+static int
+test_ptr_add_sub(void)
+{
+ int ret;
+
+ ret = test_ptr_add_sub_integer_types();
+ if (ret != 0)
+ return ret;
+
+ ret = test_ptr_add_sub_pointer_types();
+ if (ret != 0)
+ return ret;
+
+ ret = test_ptr_add_sub_typedefs();
+ if (ret != 0)
+ return ret;
+
+ return 0;
+}
+
+REGISTER_FAST_TEST(ptr_add_sub_autotest, true, true, test_ptr_add_sub);
diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
index 9e7d84f929..b525110b06 100644
--- a/lib/eal/include/rte_common.h
+++ b/lib/eal/include/rte_common.h
@@ -549,14 +549,20 @@ static void __attribute__((destructor(RTE_PRIO(prio)), used)) func(void)
/*********** Macros for pointer arithmetic ********/

/**
- * add a byte-value offset to a pointer
+ * add and subtract a byte-value offset from a pointer
*/
-#define RTE_PTR_ADD(ptr, x) ((void*)((uintptr_t)(ptr) + (x)))
-
+#ifdef RTE_CC_CLANG
/**
- * subtract a byte-value offset from a pointer
+ * Clang doesn't optimize through uintptr_t, (char*) enables
+ * optimizations and doesn't generate warnings. GCC does optimize
+ * through uintptr_t but throws warnings (e.g. array-bounds) when cast to char*.
*/
+#define RTE_PTR_ADD(ptr, x) ((void *)((char *)(uintptr_t)(ptr) + (x)))
+#define RTE_PTR_SUB(ptr, x) ((void *)((char *)(uintptr_t)(ptr) - (x)))
+#else
+#define RTE_PTR_ADD(ptr, x) ((void *)((uintptr_t)(ptr) + (x)))
#define RTE_PTR_SUB(ptr, x) ((void *)((uintptr_t)(ptr) - (x)))
+#endif

/**
* get the difference between two pointer values, i.e. how far apart
--
2.39.5 (Apple Git-154)