[PATCH v5 next 7/9] lib: mul_u64_u64_div_u64() optimise multiply on 32bit x86

David Laight
2025-11-05T20:10:58Z

gcc generates horrid code for both ((u64)u32_a * u32_b) and (u64_a + u32_b).
As well as the extra instructions it can generate a lot of spills to stack
(including spills of constant zeros and even multiplies by constant zero).

mul_u32_u32() already exists to optimise the multiply.
Add a similar add_u64_32() for the addition.
Disable both for clang - it generates better code without them.

Move the 64x64 => 128 multiply into a static inline helper function
for code clarity.
No need for the a/b_hi/lo variables, the implicit casts on the function
calls do the work for us.
Should have minimal effect on the generated code.

Use mul_u32_u32() and add_u64_u32() in the 64x64 => 128 multiply
in mul_u64_add_u64_div_u64().

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
---

Changes for v4:
- merge in patch 8.
- Add comments about gcc being 'broken' for mixed 32/64 bit maths.
clang doesn't have the same issues.
- Use a #define for define mul_add() to avoid 'defined but not used'
errors.

arch/x86/include/asm/div64.h | 19 +++++++++++++++++
include/linux/math64.h | 11 ++++++++++
lib/math/div64.c | 40 +++++++++++++++++++++++-------------
3 files changed, 56 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 6d8a3de3f43a..30fd06ede751 100644
--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -60,6 +60,12 @@ static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
}
#define div_u64_rem div_u64_rem

+/*
+ * gcc tends to zero extend 32bit values and do full 64bit maths.
+ * Define asm functions that avoid this.
+ * (clang generates better code for the C versions.)
+ */
+#ifndef __clang__
static inline u64 mul_u32_u32(u32 a, u32 b)
{
u32 high, low;
@@ -71,6 +77,19 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
}
#define mul_u32_u32 mul_u32_u32

+static inline u64 add_u64_u32(u64 a, u32 b)
+{
+ u32 high = a >> 32, low = a;
+
+ asm ("addl %[b], %[low]; adcl $0, %[high]"
+ : [low] "+r" (low), [high] "+r" (high)
+ : [b] "rm" (b) );
+
+ return low | (u64)high << 32;
+}
+#define add_u64_u32 add_u64_u32
+#endif
+
/*
* __div64_32() is never called on x86, so prevent the
* generic definition from getting built.
diff --git a/include/linux/math64.h b/include/linux/math64.h
index e889d850b7f1..cc305206d89f 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -158,6 +158,17 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
}
#endif

+#ifndef add_u64_u32
+/*
+ * Many a GCC version also messes this up.
+ * Zero extending b and then spilling everything to stack.
+ */
+static inline u64 add_u64_u32(u64 a, u32 b)
+{
+ return a + b;
+}
+#endif
+
#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)

#ifndef mul_u64_u32_shr
diff --git a/lib/math/div64.c b/lib/math/div64.c
index 18a9ba26c418..bb57a48ce36a 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -186,33 +186,45 @@ EXPORT_SYMBOL(iter_div_u64_rem);
#endif

#if !defined(mul_u64_add_u64_div_u64) || defined(test_mul_u64_add_u64_div_u64)
-u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
-{
+
+#define mul_add(a, b, c) add_u64_u32(mul_u32_u32(a, b), c)
+
#if defined(__SIZEOF_INT128__) && !defined(test_mul_u64_add_u64_div_u64)

+static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
+{
/* native 64x64=128 bits multiplication */
u128 prod = (u128)a * b + c;
- u64 n_lo = prod, n_hi = prod >> 64;
+
+ *p_lo = prod;
+ return prod >> 64;
+}

#else

- /* perform a 64x64=128 bits multiplication manually */
- u32 a_lo = a, a_hi = a >> 32, b_lo = b, b_hi = b >> 32;
+static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
+{
+ /* perform a 64x64=128 bits multiplication in 32bit chunks */
u64 x, y, z;

/* Since (x-1)(x-1) + 2(x-1) == x.x - 1 two u32 can be added to a u64 */
- x = (u64)a_lo * b_lo + (u32)c;
- y = (u64)a_lo * b_hi + (u32)(c >> 32);
- y += (u32)(x >> 32);
- z = (u64)a_hi * b_hi + (u32)(y >> 32);
- y = (u64)a_hi * b_lo + (u32)y;
- z += (u32)(y >> 32);
- x = (y << 32) + (u32)x;
-
- u64 n_lo = x, n_hi = z;
+ x = mul_add(a, b, c);
+ y = mul_add(a, b >> 32, c >> 32);
+ y = add_u64_u32(y, x >> 32);
+ z = mul_add(a >> 32, b >> 32, y >> 32);
+ y = mul_add(a >> 32, b, y);
+ *p_lo = (y << 32) + (u32)x;
+ return add_u64_u32(z, y >> 32);
+}

#endif

+u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
+{
+ u64 n_lo, n_hi;
+
+ n_hi = mul_u64_u64_add_u64(&n_lo, a, b, c);
+
if (!n_hi)
return div64_u64(n_lo, d);

--
2.39.5