Re: [PATCH 0/1] vsprintf: optimize decimal conversion (again)

Denys Vlasenko
2012-03-27T15:42:59Z

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On Tue, Mar 27, 2012 at 2:08 PM, roma1390 <roma1390@gmail.com> wrote:
> On 2012.03.26 21:47, Denys Vlasenko wrote:
>>
>> Please find test programs attached.
>>
>> 32-bit test programs were built using gcc 4.6.2
>> 64-bit test programs were built using gcc 4.2.1
>> Command line: gcc --static [-m32] -O2 -Wall test_{org,new}.c
>
> Can't compile reference for arm:
> $ arm-linux-gnueabi-gcc -O2 -Wall test_org.c -o test_org
> test_org.c: In function ‘put_dec’:
> test_org.c:101: error: impossible constraint in ‘asm’
> test_org.c:101: error: impossible constraint in ‘asm’
> test_org.c:101: error: impossible constraint in ‘asm’

Please find a modified test_header.c attached.
I tested and it builds in my arm emulator.

--
vda

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#include <features.h>
#include <inttypes.h>
#include <stdint.h>
#include <stdbool.h>
#include <sys/types.h> /* size_t */
#include <limits.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdarg.h>
#include <stddef.h>
#include <ctype.h>
#include <string.h>
#include <errno.h>
#include <stdio.h> /* for FILE */
#include <sys/time.h>
#include <time.h>

#define noinline_for_stack __attribute__((noinline))
typedef uint8_t u8;
typedef int16_t s16;

/*
* The semantics of do_div() are:
*
* uint32_t do_div(uint64_t *n, uint32_t base)
* {
* uint32_t remainder = *n % base;
* *n = *n / base;
* return remainder;
* }
*
* In other words, a 64-bit dividend with a 32-bit divisor producing
* a 64-bit result and a 32-bit remainder. To accomplish this optimally
* we call a special __do_div64 helper with completely non standard
* calling convention for arguments and results (beware).
*/

#define __asmeq(x, y) ".ifnc " x "," y " ; .err ; .endif\n\t"

#define __LINUX_ARM_ARCH__ 4

#ifdef __ARMEB__
#define __xh "r0"
#define __xl "r1"
#else
#define __xl "r0"
#define __xh "r1"
#endif

#define __do_div_asm(n, base) \
({ \
register unsigned int __base asm("r4") = base; \
register unsigned long long __n asm("r0") = n; \
register unsigned long long __res asm("r2"); \
register unsigned int __rem asm(__xh); \
asm( __asmeq("%0", __xh) \
__asmeq("%1", "r2") \
__asmeq("%2", "r0") \
__asmeq("%3", "r4") \
"bl __do_div64" \
: "=r" (__rem), "=r" (__res) \
: "r" (__n), "r" (__base) \
: "ip", "lr", "cc"); \
n = __res; \
__rem; \
})

#if __GNUC__ < 4

/*
* gcc versions earlier than 4.0 are simply too problematic for the
* optimized implementation below. First there is gcc PR 15089 that
* tend to trig on more complex constructs, spurious .global __udivsi3
* are inserted even if none of those symbols are referenced in the
* generated code, and those gcc versions are not able to do constant
* propagation on long long values anyway.
*/
#define do_div(n, base) __do_div_asm(n, base)

#elif __GNUC__ >= 4

/*
* If the divisor happens to be constant, we determine the appropriate
* inverse at compile time to turn the division into a few inline
* multiplications instead which is much faster. And yet only if compiling
* for ARMv4 or higher (we need umull/umlal) and if the gcc version is
* sufficiently recent to perform proper long long constant propagation.
* (It is unfortunate that gcc doesn't perform all this internally.)
*/
#define do_div(n, base) \
({ \
unsigned int __r, __b = (base); \
if (!__builtin_constant_p(__b) || __b == 0 || \
(__LINUX_ARM_ARCH__ < 4 && (__b & (__b - 1)) != 0)) { \
/* non-constant divisor (or zero): slow path */ \
__r = __do_div_asm(n, __b); \
} else if ((__b & (__b - 1)) == 0) { \
/* Trivial: __b is constant and a power of 2 */ \
/* gcc does the right thing with this code. */ \
__r = n; \
__r &= (__b - 1); \
n /= __b; \
} else { \
/* Multiply by inverse of __b: n/b = n*(p/b)/p */ \
/* We rely on the fact that most of this code gets */ \
/* optimized away at compile time due to constant */ \
/* propagation and only a couple inline assembly */ \
/* instructions should remain. Better avoid any */ \
/* code construct that might prevent that. */ \
unsigned long long __res, __x, __t, __m, __n = n; \
unsigned int __c, __p, __z = 0; \
/* preserve low part of n for reminder computation */ \
__r = __n; \
/* determine number of bits to represent __b */ \
__p = 1 << __div64_fls(__b); \
/* compute __m = ((__p << 64) + __b - 1) / __b */ \
__m = (~0ULL / __b) * __p; \
__m += (((~0ULL % __b + 1) * __p) + __b - 1) / __b; \
/* compute __res = __m*(~0ULL/__b*__b-1)/(__p << 64) */ \
__x = ~0ULL / __b * __b - 1; \
__res = (__m & 0xffffffff) * (__x & 0xffffffff); \
__res >>= 32; \
__res += (__m & 0xffffffff) * (__x >> 32); \
__t = __res; \
__res += (__x & 0xffffffff) * (__m >> 32); \
__t = (__res < __t) ? (1ULL << 32) : 0; \
__res = (__res >> 32) + __t; \
__res += (__m >> 32) * (__x >> 32); \
__res /= __p; \
/* Now sanitize and optimize what we've got. */ \
if (~0ULL % (__b / (__b & -__b)) == 0) { \
/* those cases can be simplified with: */ \
__n /= (__b & -__b); \
__m = ~0ULL / (__b / (__b & -__b)); \
__p = 1; \
__c = 1; \
} else if (__res != __x / __b) { \
/* We can't get away without a correction */ \
/* to compensate for bit truncation errors. */ \
/* To avoid it we'd need an additional bit */ \
/* to represent __m which would overflow it. */ \
/* Instead we do m=p/b and n/b=(n*m+m)/p. */ \
__c = 1; \
/* Compute __m = (__p << 64) / __b */ \
__m = (~0ULL / __b) * __p; \
__m += ((~0ULL % __b + 1) * __p) / __b; \
} else { \
/* Reduce __m/__p, and try to clear bit 31 */ \
/* of __m when possible otherwise that'll */ \
/* need extra overflow handling later. */ \
unsigned int __bits = -(__m & -__m); \
__bits |= __m >> 32; \
__bits = (~__bits) << 1; \
/* If __bits == 0 then setting bit 31 is */ \
/* unavoidable. Simply apply the maximum */ \
/* possible reduction in that case. */ \
/* Otherwise the MSB of __bits indicates the */ \
/* best reduction we should apply. */ \
if (!__bits) { \
__p /= (__m & -__m); \
__m /= (__m & -__m); \
} else { \
__p >>= __div64_fls(__bits); \
__m >>= __div64_fls(__bits); \
} \
/* No correction needed. */ \
__c = 0; \
} \
/* Now we have a combination of 2 conditions: */ \
/* 1) whether or not we need a correction (__c), and */ \
/* 2) whether or not there might be an overflow in */ \
/* the cross product (__m & ((1<<63) | (1<<31))) */ \
/* Select the best insn combination to perform the */ \
/* actual __m * __n / (__p << 64) operation. */ \
if (!__c) { \
asm ( "umull %Q0, %R0, %1, %Q2\n\t" \
"mov %Q0, #0" \
: "=&r" (__res) \
: "r" (__m), "r" (__n) \
: "cc" ); \
} else if (!(__m & ((1ULL << 63) | (1ULL << 31)))) { \
__res = __m; \
asm ( "umlal %Q0, %R0, %Q1, %Q2\n\t" \
"mov %Q0, #0" \
: "+&r" (__res) \
: "r" (__m), "r" (__n) \
: "cc" ); \
} else { \
asm ( "umull %Q0, %R0, %Q1, %Q2\n\t" \
"cmn %Q0, %Q1\n\t" \
"adcs %R0, %R0, %R1\n\t" \
"adc %Q0, %3, #0" \
: "=&r" (__res) \
: "r" (__m), "r" (__n), "r" (__z) \
: "cc" ); \
} \
if (!(__m & ((1ULL << 63) | (1ULL << 31)))) { \
asm ( "umlal %R0, %Q0, %R1, %Q2\n\t" \
"umlal %R0, %Q0, %Q1, %R2\n\t" \
"mov %R0, #0\n\t" \
"umlal %Q0, %R0, %R1, %R2" \
: "+&r" (__res) \
: "r" (__m), "r" (__n) \
: "cc" ); \
} else { \
asm ( "umlal %R0, %Q0, %R2, %Q3\n\t" \
"umlal %R0, %1, %Q2, %R3\n\t" \
"mov %R0, #0\n\t" \
"adds %Q0, %1, %Q0\n\t" \
"adc %R0, %R0, #0\n\t" \
"umlal %Q0, %R0, %R2, %R3" \
: "+&r" (__res), "+&r" (__z) \
: "r" (__m), "r" (__n) \
: "cc" ); \
} \
__res /= __p; \
/* The reminder can be computed with 32-bit regs */ \
/* only, and gcc is good at that. */ \
{ \
unsigned int __res0 = __res; \
unsigned int __b0 = __b; \
__r -= __res0 * __b0; \
} \
/* BUG_ON(__r >= __b || __res * __b + __r != n); */ \
n = __res; \
} \
__r; \
})

/* our own fls implementation to make sure constant propagation is fine */
#define __div64_fls(bits) \
({ \
unsigned int __left = (bits), __nr = 0; \
if (__left & 0xffff0000) __nr += 16, __left >>= 16; \
if (__left & 0x0000ff00) __nr += 8, __left >>= 8; \
if (__left & 0x000000f0) __nr += 4, __left >>= 4; \
if (__left & 0x0000000c) __nr += 2, __left >>= 2; \
if (__left & 0x00000002) __nr += 1; \
__nr; \
})

#endif