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[libc] Add inline assembly implementation of flsl() using BSR instruction

Signed-off-by: Michael Brown <mcb30@ipxe.org>
tags/v1.20.1
Michael Brown преди 10 години
родител
ревизия
dce7107fc0
променени са 3 файла, в които са добавени 33 реда и са изтрити 15 реда
  1. 31
    0
      src/arch/x86/include/bits/strings.h
  2. 0
    13
      src/core/bitops.c
  3. 2
    2
      src/include/strings.h

+ 31
- 0
src/arch/x86/include/bits/strings.h Целия файл

@@ -0,0 +1,31 @@
1
+#ifndef _BITS_STRINGS_H
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+#define _BITS_STRINGS_H
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+
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+FILE_LICENCE ( GPL2_OR_LATER );
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+
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+/**
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+ * Find last (i.e. most significant) set bit
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+ *
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+ * @v value		Value
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+ * @ret msb		Most significant bit set in value (LSB=1), or zero
11
+ */
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+static inline __attribute__ (( always_inline )) int __flsl ( long value ) {
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+	long msb_minus_one;
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+
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+	/* If the input value is zero, the BSR instruction returns
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+	 * ZF=1 and leaves an undefined value in the output register.
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+	 * Perform this check in C rather than asm so that it can be
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+	 * omitted in cases where the compiler is able to prove that
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+	 * the input is non-zero.
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+	 */
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+	if ( value ) {
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+		__asm__ ( "bsr %1, %0"
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+			  : "=r" ( msb_minus_one )
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+			  : "rm" ( value ) );
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+		return ( msb_minus_one + 1 );
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+	} else {
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+		return 0;
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+	}
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+}
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+
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+#endif /* _BITS_STRINGS_H */

+ 0
- 13
src/core/bitops.c Целия файл

@@ -1,13 +0,0 @@
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-#include <strings.h>
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-
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-FILE_LICENCE ( GPL2_OR_LATER );
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-
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-int __flsl ( long x ) {
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-	unsigned long value = x;
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-	int ls = 0;
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-
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-	for ( ls = 0 ; value ; ls++ ) {
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-		value >>= 1;
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-	}
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-	return ls;
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-}

+ 2
- 2
src/include/strings.h Целия файл

@@ -5,6 +5,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
5 5
 
6 6
 #include <limits.h>
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 #include <string.h>
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+#include <bits/strings.h>
8 9
 
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 static inline __attribute__ (( always_inline )) int
10 11
 __constant_flsl ( unsigned long x ) {
@@ -42,8 +43,7 @@ __constant_flsl ( unsigned long x ) {
42 43
 	return r;
43 44
 }
44 45
 
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-/* We don't actually have these functions yet */
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-extern int __flsl ( long x );
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+int __flsl ( long x );
47 47
 
48 48
 #define flsl( x ) \
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 	( __builtin_constant_p ( x ) ? __constant_flsl ( x ) : __flsl ( x ) )

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