xen/bitops: Implement ffs() in common logic

Perform constant-folding unconditionally, rather than having it implemented
inconsistency between architectures.

Confirm the expected behaviour with compile time and boot time tests.

For non-constant inputs, use arch_ffs() if provided but fall back to
generic_ffsl() if not.  In particular, RISC-V doesn't have a builtin that
works in all configurations.

For x86, rename ffs() to arch_ffs() and adjust the prototype.

For PPC, __builtin_ctz() is 1/3 of the size of size of the transform to
generic_fls().  Drop the definition entirely.  ARM too benefits in the general
case by using __builtin_ctz(), but less dramatically because it using
optimised asm().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/arm/include/asm/bitops.h

@@ -157,7 +157,7 @@ static inline int fls(unsigned int x)
 }
 
 
-#define ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); })
+#define arch_ffs(x)  ((x) ? 1 + __builtin_ctz(x) : 0)
 #define ffsl(x) ({ unsigned long __t = (x); flsl(ISOLATE_LSB(__t)); })
 
 /**

xen/arch/ppc/include/asm/bitops.h

@@ -173,7 +173,7 @@ static inline int __test_and_clear_bit(int nr, volatile void *addr)
 
 #define flsl(x) generic_flsl(x)
 #define fls(x) generic_flsl(x)
-#define ffs(x) ({ unsigned int t_ = (x); fls(t_ & -t_); })
+#define arch_ffs(x)  ((x) ? 1 + __builtin_ctz(x) : 0)
 #define ffsl(x) ({ unsigned long t_ = (x); flsl(t_ & -t_); })
 
 /**

xen/arch/x86/include/asm/bitops.h

@@ -418,7 +418,7 @@ static inline int ffsl(unsigned long x)
     return (int)r+1;
 }
 
-static inline int ffs(unsigned int x)
+static always_inline unsigned int arch_ffs(unsigned int x)
 {
     int r;
 
@@ -428,6 +428,7 @@ static inline int ffs(unsigned int x)
           "1:" : "=r" (r) : "rm" (x));
     return r + 1;
 }
+#define arch_ffs arch_ffs
 
 /**
  * fls - find last bit set

xen/common/Makefile

@@ -1,5 +1,6 @@
 obj-$(CONFIG_ARGO) += argo.o
 obj-y += bitmap.o
+obj-bin-$(CONFIG_SELF_TESTS) += bitops.init.o
 obj-$(CONFIG_GENERIC_BUG_FRAME) += bug.o
 obj-$(CONFIG_HYPFS_CONFIG) += config_data.o
 obj-$(CONFIG_CORE_PARKING) += core_parking.o

xen/common/bitops.c

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <xen/bitops.h>
+#include <xen/init.h>
+#include <xen/self-tests.h>
+
+static void __init test_ffs(void)
+{
+    /* unsigned int ffs(unsigned int) */
+    CHECK(ffs, 0, 0);
+    CHECK(ffs, 1, 1);
+    CHECK(ffs, 3, 1);
+    CHECK(ffs, 7, 1);
+    CHECK(ffs, 6, 2);
+    CHECK(ffs, 0x80000000U, 32);
+}
+
+static void __init __constructor test_bitops(void)
+{
+    test_ffs();
+}

xen/include/xen/bitops.h

@@ -31,6 +31,23 @@ unsigned int __pure generic_flsl(unsigned long x);
 
 #include <asm/bitops.h>
 
+/*
+ * Find First/Last Set bit (all forms).
+ *
+ * Bits are labelled from 1.  Returns 0 if given 0.
+ */
+static always_inline __pure unsigned int ffs(unsigned int x)
+{
+    if ( __builtin_constant_p(x) )
+        return __builtin_ffs(x);
+
+#ifdef arch_ffs
+    return arch_ffs(x);
+#else
+    return generic_ffsl(x);
+#endif
+}
+
 /* --------------------- Please tidy below here --------------------- */
 
 #ifndef find_next_bit