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

H. Peter Anvin
2025-11-05T23:46:58Z

On November 5, 2025 12:10:33 PM PST, David Laight <david.laight.linux@gmail.com> wrote:
>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);
>

By the way have you filed gcc bug reports for this?