linux-kernel - [PATCH] sparc/vdso: Add helper function for 64-bit right shift on 32-bit target

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Message-Id: <20240804-sparc-shr64-v1-1-25050968339a@protonmail.com>
Date: Sun, 04 Aug 2024 10:39:49 +0700
From: Koakuma via B4 Relay <devnull+koachan.protonmail.com@...nel.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Andreas Larsson <andreas@...sler.com>, Andy Lutomirski <luto@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>, 
 Vincenzo Frascino <vincenzo.frascino@....com>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Cc: sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, Koakuma <koachan@...tonmail.com>
Subject: [PATCH] sparc/vdso: Add helper function for 64-bit right shift on
 32-bit target

From: Koakuma <koachan@...tonmail.com>

Add helper function for 64-bit right shift on 32-bit target so that
clang does not emit a runtime library call.

Signed-off-by: Koakuma <koachan@...tonmail.com>
---
Hi~

This adds a small function to do 64-bit right shifts for use in vDSO
code, needed so that clang does not emit a call to runtime library.
---
 arch/sparc/vdso/vclock_gettime.c |  8 ++++----
 include/vdso/math64.h            | 28 ++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/arch/sparc/vdso/vclock_gettime.c b/arch/sparc/vdso/vclock_gettime.c
index e794edde6755..c0251a632bdb 100644
--- a/arch/sparc/vdso/vclock_gettime.c
+++ b/arch/sparc/vdso/vclock_gettime.c
@@ -154,7 +154,7 @@ notrace static __always_inline int do_realtime(struct vvar_data *vvar,
 		ts->tv_sec = vvar->wall_time_sec;
 		ns = vvar->wall_time_snsec;
 		ns += vgetsns(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -174,7 +174,7 @@ notrace static __always_inline int do_realtime_stick(struct vvar_data *vvar,
 		ts->tv_sec = vvar->wall_time_sec;
 		ns = vvar->wall_time_snsec;
 		ns += vgetsns_stick(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -194,7 +194,7 @@ notrace static __always_inline int do_monotonic(struct vvar_data *vvar,
 		ts->tv_sec = vvar->monotonic_time_sec;
 		ns = vvar->monotonic_time_snsec;
 		ns += vgetsns(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -214,7 +214,7 @@ notrace static __always_inline int do_monotonic_stick(struct vvar_data *vvar,
 		ts->tv_sec = vvar->monotonic_time_sec;
 		ns = vvar->monotonic_time_snsec;
 		ns += vgetsns_stick(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
diff --git a/include/vdso/math64.h b/include/vdso/math64.h
index 22ae212f8b28..771d84faa8d7 100644
--- a/include/vdso/math64.h
+++ b/include/vdso/math64.h
@@ -21,6 +21,34 @@ __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
 	return ret;
 }
 
+#if BITS_PER_LONG == 32
+/* This is to prevent the compiler from emitting a call to __lshrdi3. */
+static __always_inline u64
+__shr64(u64 val, int amt)
+{
+	u32 mask = (1U << amt) - 1;
+	u32 lo = val;
+	u32 hi = val >> 32;
+	u32 mi;
+
+	if (amt >= 32)
+		return hi >> (amt - 32);
+
+
+	mi = (hi & mask) << (32 - amt);
+	hi >>= amt;
+	lo = (lo >> amt) | mi;
+
+	return ((u64) hi) << 32 | lo;
+}
+#else
+static __always_inline u64
+__shr64(u64 val, int amt)
+{
+	return val >> amt;
+}
+#endif /* BITS_PER_LONG == 32 */
+
 #if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
 
 #ifndef mul_u64_u32_add_u64_shr

---
base-commit: defaf1a2113a22b00dfa1abc0fd2014820eaf065
change-id: 20240717-sparc-shr64-2f00a7884770

Best regards,
-- 
Koakuma <koachan@...tonmail.com>