linux-kernel - [PATCH v4 3/4] ARM: div64: improve __arch_xprod

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Message-ID: <20241003211829.2750436-4-nico@fluxnic.net>
Date: Thu,  3 Oct 2024 17:16:15 -0400
From: Nicolas Pitre <nico@...xnic.net>
To: Arnd Bergmann <arnd@...db.de>,
	Russell King <linux@...linux.org.uk>
Cc: Nicolas Pitre <npitre@...libre.com>,
	linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/4] ARM: div64: improve __arch_xprod_64()

From: Nicolas Pitre <npitre@...libre.com>

Let's use the same criteria for overflow handling necessity as the
generic code.

Signed-off-by: Nicolas Pitre <npitre@...libre.com>
---
 arch/arm/include/asm/div64.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
index 4b69cf8504..562d5376ae 100644
--- a/arch/arm/include/asm/div64.h
+++ b/arch/arm/include/asm/div64.h
@@ -56,6 +56,8 @@ static inline uint64_t __arch_xprod_64(uint64_t m, uint64_t n, bool bias)
 {
 	unsigned long long res;
 	register unsigned int tmp asm("ip") = 0;
+	bool no_ovf = __builtin_constant_p(m) &&
+		      ((m >> 32) + (m & 0xffffffff) < 0x100000000);
 
 	if (!bias) {
 		asm (	"umull	%Q0, %R0, %Q1, %Q2\n\t"
@@ -63,7 +65,7 @@ static inline uint64_t __arch_xprod_64(uint64_t m, uint64_t n, bool bias)
 			: "=&r" (res)
 			: "r" (m), "r" (n)
 			: "cc");
-	} else if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
+	} else if (no_ovf) {
 		res = m;
 		asm (	"umlal	%Q0, %R0, %Q1, %Q2\n\t"
 			"mov	%Q0, #0"
@@ -80,7 +82,7 @@ static inline uint64_t __arch_xprod_64(uint64_t m, uint64_t n, bool bias)
 			: "cc");
 	}
 
-	if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
+	if (no_ovf) {
 		asm (	"umlal	%R0, %Q0, %R1, %Q2\n\t"
 			"umlal	%R0, %Q0, %Q1, %R2\n\t"
 			"mov	%R0, #0\n\t"
-- 
2.46.1