[PATCH v3 0/2] riscv: lib: add optimized memcmp() and extend KUnit tests

[PATCH v3 0/2] riscv: lib: add optimized memcmp() and extend KUnit tests

[Milan Tripkovic]

This v3 series introduces an optimized RISC-V memcmp() implementation and
extends the KUnit string tests to cover functional correctness and
benchmarking of memcmp().

The memcmp() implementation incorporates review feedback from earlier
versions, improving alignment handling, loop structure, Zbb usage, and
overall correctness.

The KUnit updates add comprehensive memcmp() test coverage and a benchmark
mode, with v3 addressing structural cleanups and style issues raised during
review.

Signed-off-by: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>

---
v3 changes:
    - Split memcmp benchmark into wrapper (string_bench_memcmp) and worker
      function (do_string_bench_memcmp).
    - Removed all C99 mixed declarations; moved all variable declarations
      to the top of each function.
    - Converted len, iterations and loop counters in the benchmark to u64
      to avoid implicit casts.
    - Cleaned up spacing, indentation and minor style issues.
    - Added #if defined(CONFIG_RISCV_ISA_ZBB)... in  memcmp.S
    - Link to v2:https://lore.kernel.org/all/20260514121359.931999-1-milant2002@gmail.com/
		
v2 changes:
    - Added alignment checks for buffers to avoid expensive misaligned loads.
    - Optimized the loop using end-pointers to reduce per-iteration overhead.
    - Implemented word-aligned tail handling using ZBB shifts.
    - Removed redundant pointer equality (a0 == a1) check.
    - Retained BE support via #ifndef; ZBB rev8 is used for the LE fast-path.
    - Fixed KUnit build failures for Clang and non-benchmark configs.
    - Link to v1:https://lore.kernel.org/all/20260512141007.1193033-1-milant2002@gmail.com/
Milan Tripkovic (2):
  riscv: lib: add memcmp() implementation
  lib/string_kunit: extend benchmarks and unit test to memcmp()

 arch/riscv/include/asm/string.h |   2 +
 arch/riscv/lib/Makefile         |   1 +
 arch/riscv/lib/memcmp.S         | 125 ++++++++++++++++++++++++++++++++
 arch/riscv/purgatory/Makefile   |   5 +-
 lib/tests/string_kunit.c        | 116 +++++++++++++++++++++++++++++
 5 files changed, 248 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/lib/memcmp.S

-- 
2.43.0


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[PATCH v3 1/2] riscv: lib: add memcmp() implementation

[Milan Tripkovic]

From: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>

Add an assembly implementation of memcmp() for RISC-V. The implementation
uses the ZBB extension for word-at-a-time comparison and an assembly
fallback for non-ZBB systems.

Benchmark results (QEMU TCG, rv64, Aligned):

  Len   | Default | NoZBB  | ZBB    | %NoZBB | %ZBB
  ------|---------|--------|--------|--------|-------
  1 B   | 20.3    | 25.0   | 20.9   | +23.2% | +3.0%
  7 B   | 88.9    | 107.5  | 155.7  | +20.9% | +75.1%
  8 B   | 89.6    | 110.9  | 176.2  | +23.8% | +96.7%
  16 B  | 134.4   | 172.4  | 334.8  | +28.3% | +149.1%
  31 B  | 163.5   | 220.5  | 606.2  | +34.9% | +270.8%
  64 B  | 203.8   | 235.9  | 968.6  | +15.8% | +375.3%
  127 B | 224.6   | 268.7  | 1362.8 | +19.6% | +506.8%
  512 B | 235.7   | 271.1  | 1913.7 | +15.0% | +711.9%
  1024 B| 256.8   | 290.6  | 2123.6 | +13.2% | +726.9%
  4096 B| 263.8   | 302.9  | 2290.4 | +14.8% | +768.2%

Benchmark results (QEMU TCG, rv64, Unaligned - Offset 3):

  Len   | Default | NoZBB  | ZBB    | %NoZBB | %ZBB
  ------|---------|--------|--------|--------|-------
  1 B   | 20.7    | 21.7   | 21.5   | +4.8%  | +3.9%
  7 B   | 96.2    | 99.1   | 96.9   | +3.0%  | +0.7%
  8 B   | 97.5    | 118.5  | 110.5  | +21.5% | +13.3%
  16 B  | 136.7   | 166.6  | 172.8  | +21.9% | +26.4%
  31 B  | 167.6   | 206.5  | 211.9  | +23.2% | +26.4%
  64 B  | 204.4   | 229.9  | 240.3  | +12.5% | +17.6%
  127 B | 229.6   | 261.7  | 269.0  | +14.0% | +17.2%
  512 B | 245.5   | 260.8  | 269.9  | +6.2%  | +9.9%
  1024 B| 246.9   | 261.2  | 283.5  | +5.8%  | +14.8%
  4096 B| 250.7   | 295.8  | 299.7  | +18.0% | +19.5%

Signed-off-by: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>
---
 arch/riscv/include/asm/string.h |   2 +
 arch/riscv/lib/Makefile         |   1 +
 arch/riscv/lib/memcmp.S         | 125 ++++++++++++++++++++++++++++++++
 arch/riscv/purgatory/Makefile   |   5 +-
 4 files changed, 132 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/lib/memcmp.S

diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
index 764ffe8f6..5c5299678 100644
--- a/arch/riscv/include/asm/string.h
+++ b/arch/riscv/include/asm/string.h
@@ -18,6 +18,8 @@ extern asmlinkage void *__memcpy(void *, const void *, size_t);
 #define __HAVE_ARCH_MEMMOVE
 extern asmlinkage void *memmove(void *, const void *, size_t);
 extern asmlinkage void *__memmove(void *, const void *, size_t);
+#define __HAVE_ARCH_MEMCMP
+extern asmlinkage int memcmp(const void *, const void *, size_t);
 
 #if !(defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
 #define __HAVE_ARCH_STRCMP
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index 6f767b2a3..b529e1be1 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -3,6 +3,7 @@ lib-y			+= delay.o
 lib-y			+= memcpy.o
 lib-y			+= memset.o
 lib-y			+= memmove.o
+lib-y			+= memcmp.o
 ifeq ($(CONFIG_KASAN_GENERIC)$(CONFIG_KASAN_SW_TAGS),)
 lib-y			+= strcmp.o
 lib-y			+= strlen.o
diff --git a/arch/riscv/lib/memcmp.S b/arch/riscv/lib/memcmp.S
new file mode 100644
index 000000000..a531e481c
--- /dev/null
+++ b/arch/riscv/lib/memcmp.S
@@ -0,0 +1,125 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <linux/linkage.h>
+#include <asm/asm.h>
+#include <asm/alternative-macros.h>
+#include <asm/hwcap.h>
+
+/* int memcmp(const void *cs, const void *ct, size_t n) */
+SYM_FUNC_START(memcmp)
+
+	__ALTERNATIVE_CFG("nop", "j memcmp_zbb", 0, RISCV_ISA_EXT_ZBB,
+		IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB))
+/*
+ * Parameters
+ *	a0 - Pointer to first memory block (cs), also return value
+ *	a1 - Pointer to second memory block (ct)
+ *	a2 - Number of bytes to compare (n), transformed to end pointer (a0 + n)
+ *
+ * Returns
+ *	a0 - 0 if equal, positive if cs > ct, negative if cs < ct
+ *
+ * Clobbers
+ *	t0, t1
+ */
+	beqz	a2, 2f
+	add	a2, a0, a2
+1:
+	lbu	t0, 0(a0)
+	lbu	t1, 0(a1)
+	bne	t0, t1, 3f
+	addi	a0, a0, 1
+	addi	a1, a1, 1
+	bne	a0, a2, 1b
+2:
+	li	a0, 0
+	ret
+3:
+	sub	a0, t0, t1
+	ret
+
+#if defined(CONFIG_RISCV_ISA_ZBB) && defined(CONFIG_TOOLCHAIN_HAS_ZBB)
+memcmp_zbb:
+
+.option push
+.option arch,+zbb
+/*
+ * Parameters
+ *	a0 - Pointer to first memory block (cs), also return value
+ *	a1 - Pointer to second memory block (ct)
+ *	a2 - Number of bytes to compare (n), decremented during loop
+ *
+ * Returns
+ *	a0 - 0 if equal, positive if cs > ct, negative if cs < ct
+ *
+ * Clobbers
+ *	t0, t1, t2, t3, t4
+ */
+	add	t3, a0, a2
+	or	t0, a0, a1
+	andi	t0, t0, (SZREG - 1)
+	bnez	t0, 5f
+
+	addi	t4, t3, -SZREG
+	bltu	t4, a0, 7f
+
+1:
+	REG_L	t1, 0(a0)
+	REG_L	t2, 0(a1)
+	bne	t1, t2, 2f
+	addi	a0, a0, SZREG
+	addi	a1, a1, SZREG
+	bleu	a0, t4, 1b
+
+7:
+	beq	a0, t3, 4f
+	REG_L	t1, 0(a0)
+	REG_L	t2, 0(a1)
+
+	sub	t0, t3, a0
+	li	t4, SZREG
+	sub	t0, t4, t0
+	slli	t0, t0, 3
+
+#ifndef CONFIG_CPU_BIG_ENDIAN
+	rev8	t1, t1
+	rev8	t2, t2
+#endif
+	srl	t1, t1, t0
+	srl	t2, t2, t0
+
+	bne	t1, t2, 8f
+	li	a0, 0
+	ret
+5:
+	beq	a0, t3, 4f
+6:
+	lbu	t1, 0(a0)
+	lbu	t2, 0(a1)
+	bne	t1, t2, 3f
+	addi	a0, a0, 1
+	addi	a1, a1, 1
+	bne	a0, t3, 6b
+
+4:	li	a0, 0
+	ret
+2:
+#ifndef CONFIG_CPU_BIG_ENDIAN
+	rev8	t1, t1
+	rev8	t2, t2
+#endif
+8:
+	sltu	a0, t2, t1
+	sltu	t0, t1, t2
+	sub	a0, a0, t0
+	ret
+
+3:
+	sub	a0, t1, t2
+	ret
+
+.option pop
+#endif
+SYM_FUNC_END(memcmp)
+SYM_FUNC_ALIAS(__pi_memcmp, memcmp)
+EXPORT_SYMBOL(memcmp)
diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index b0358a78f..456929971 100644
--- a/arch/riscv/purgatory/Makefile
+++ b/arch/riscv/purgatory/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
-purgatory-y := purgatory.o sha256.o entry.o string.o ctype.o memcpy.o memset.o
+purgatory-y := purgatory.o sha256.o entry.o string.o ctype.o memcpy.o memset.o memcmp.o
 ifeq ($(CONFIG_KASAN_GENERIC)$(CONFIG_KASAN_SW_TAGS),)
 purgatory-y += strcmp.o strlen.o strncmp.o strnlen.o strchr.o strrchr.o
 endif
@@ -41,6 +41,9 @@ $(obj)/strchr.o: $(srctree)/arch/riscv/lib/strchr.S FORCE
 $(obj)/strrchr.o: $(srctree)/arch/riscv/lib/strrchr.S FORCE
 	$(call if_changed_rule,as_o_S)
 
+$(obj)/memcmp.o: $(srctree)/arch/riscv/lib/memcmp.S FORCE
+	$(call if_changed_rule,as_o_S)
+
 CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
 CFLAGS_string.o := -D__DISABLE_EXPORTS
 CFLAGS_ctype.o := -D__DISABLE_EXPORTS
-- 
2.43.0


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[PATCH v3 2/2] lib/string_kunit: extend benchmarks and unit test to memcmp()

[Milan Tripkovic]

From: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>

Extend the string benchmarking suite to include memcmp().
Extend the string unit test to include memcmp().

Signed-off-by: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>
---
 lib/tests/string_kunit.c | 116 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)

diff --git a/lib/tests/string_kunit.c b/lib/tests/string_kunit.c
index 0819ace5b..95d65c25b 100644
--- a/lib/tests/string_kunit.c
+++ b/lib/tests/string_kunit.c
@@ -881,6 +881,120 @@ static void string_bench_strrchr(struct kunit *test)
 	STRING_BENCH_BUF(test, buf, len, strrchr, buf, '\0');
 }
 
+static void string_test_memcmp(struct kunit *test)
+{
+	const unsigned int max_offset = 16;
+	const unsigned int max_len = 32;
+	const unsigned int buf_size = max_offset + max_len + 32;
+	u8 *buf1, *buf2;
+	unsigned int i, j, len, k;
+	int res;
+
+	buf1 = kunit_kzalloc(test, buf_size, GFP_KERNEL);
+	buf2 = kunit_kzalloc(test, buf_size, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buf1);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buf2);
+
+	for (i = 0; i < max_offset; i++) {
+		for (j = 0; j < max_offset; j++) {
+			for (len = 0; len <= max_len; len++) {
+				memset(buf1, 'A', buf_size);
+				memset(buf2, 'A', buf_size);
+				KUNIT_EXPECT_EQ_MSG(test, memcmp(buf1 + i, buf2 + j, len), 0,
+						    "Should be equal: i:%u j:%u len:%u", i, j, len);
+				for (k = 0; k < len; k++) {
+					memset(buf1, 'A', buf_size);
+					memset(buf2, 'A', buf_size);
+					buf2[j + k] = 'B';
+					res = memcmp(buf1 + i, buf2 + j, len);
+					KUNIT_EXPECT_NE_MSG(test, res, 0,
+							    "Should detect difference at k:%u (i:%u j:%u len:%u)",
+						k, i, j, len);
+					if (buf1[i + k] < buf2[j + k])
+						KUNIT_EXPECT_LT(test, res, 0);
+					else
+						KUNIT_EXPECT_GT(test, res, 0);
+				}
+			}
+		}
+	}
+}
+
+static void do_string_bench_memcmp(struct kunit *test)
+{
+	char *buf1 = NULL;
+	char *buf2 = NULL;
+	const u64 lengths[] = { 1, 7, 8, 16, 32, 64, 128, 512, 1024, 4096 };
+	const int offsets[] = { 0, 1, 3, 7 };
+	const u64 max_len = 4096 + 64;
+	unsigned int w, o, i;
+	unsigned int off;
+	u64 len;
+	char *p1;
+	char *p2;
+	u64 iterations;
+	u64 elapsed;
+	u64 ns_per_call;
+	u64 mbps;
+	u64 j;
+
+	buf1 = vmalloc(max_len);
+	buf2 = vmalloc(max_len);
+
+	if (!buf1 || !buf2) {
+		vfree(buf1);
+		vfree(buf2);
+		kunit_err(test, "vmalloc failed\n");
+		return;
+	}
+
+	memset(buf1, 'A', max_len);
+	memset(buf2, 'A', max_len);
+
+	for (w = 0; w < 100000U; w++)
+		(void)memcmp(buf1, buf2, 4096);
+
+	for (o = 0; o < ARRAY_SIZE(offsets); o++) {
+		off = offsets[o];
+
+		for (i = 0; i < ARRAY_SIZE(lengths); i++) {
+			len = lengths[i];
+			p1 = buf1;
+			p2 = buf2 + off;
+			iterations = (len < 512) ? 100000ULL : 10000ULL;
+
+			for (j = 0; j < iterations; j++) {
+				(void)memcmp(p1, p2, len);
+				barrier();
+			}
+
+			elapsed = STRING_BENCH(iterations, memcmp, p1, p2, len);
+			ns_per_call = div_u64(elapsed, iterations);
+			mbps = len ? div_u64(iterations * len * (NSEC_PER_SEC / MEGA), elapsed) : 0;
+
+			if (off == 0) {
+				kunit_info(test, "bench_memcmp_aligned: len=%-4llu: %llu MB/s (%llu ns/call)\n",
+					   len, mbps, ns_per_call);
+			} else {
+				kunit_info(test, "bench_memcmp_unaligned(off=%u): len=%-4llu: %llu MB/s (%llu ns/call)\n",
+					   off, len, mbps, ns_per_call);
+			}
+		}
+	}
+
+	vfree(buf1);
+	vfree(buf2);
+}
+
+static void string_bench_memcmp(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_STRING_KUNIT_BENCH)) {
+		kunit_skip(test, "CONFIG_STRING_KUNIT_BENCH not enabled");
+		return;
+	}
+	do_string_bench_memcmp(test);
+}
+
 static struct kunit_case string_test_cases[] = {
 	KUNIT_CASE(string_test_memset16),
 	KUNIT_CASE(string_test_memset32),
@@ -910,6 +1024,8 @@ static struct kunit_case string_test_cases[] = {
 	KUNIT_CASE(string_bench_strnlen),
 	KUNIT_CASE(string_bench_strchr),
 	KUNIT_CASE(string_bench_strrchr),
+	KUNIT_CASE(string_test_memcmp),
+	KUNIT_CASE_SLOW(string_bench_memcmp),
 	{}
 };
 
-- 
2.43.0


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