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>
---

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 #defdine 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 cabdc2d5a68f..a18c045aa8a1 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 f92e7160feb6..f6da7b5fb69e 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 (unlikely(n_hi >= d)) {
 		/* trigger runtime exception if divisor is zero */
 		if (d == 0) {
-- 
2.39.5

On Wed, 29 Oct 2025, David Laight 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 #defdine 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 cabdc2d5a68f..a18c045aa8a1 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 f92e7160feb6..f6da7b5fb69e 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 (unlikely(n_hi >= d)) {
>  		/* trigger runtime exception if divisor is zero */
>  		if (d == 0) {
> -- 
> 2.39.5
> 
>