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      1 // This file is dual licensed under the MIT and the University of Illinois Open
      2 // Source Licenses. See LICENSE.TXT for details.
      3 
      4 #include "../assembly.h"
      5 
      6 // du_int __umoddi3(du_int a, du_int b);
      7 
      8 // result = remainder of a / b.
      9 // both inputs and the output are 64-bit unsigned integers.
     10 // This will do whatever the underlying hardware is set to do on division by zero.
     11 // No other exceptions are generated, as the divide cannot overflow.
     12 //
     13 // This is targeted at 32-bit x86 *only*, as this can be done directly in hardware
     14 // on x86_64.  The performance goal is ~40 cycles per divide, which is faster than
     15 // currently possible via simulation of integer divides on the x87 unit.
     16 //
     17 
     18 // Stephen Canon, December 2008
     19 
     20 #ifdef __i386__
     21 
     22 .text
     23 .balign 4
     24 DEFINE_COMPILERRT_FUNCTION(__umoddi3)
     25 
     26 	pushl		%ebx
     27 	movl	 20(%esp),			%ebx	// Find the index i of the leading bit in b.
     28 	bsrl		%ebx,			%ecx	// If the high word of b is zero, jump to
     29 	jz			9f						// the code to handle that special case [9].
     30 
     31 	/* High word of b is known to be non-zero on this branch */
     32 
     33 	movl	 16(%esp),			%eax	// Construct bhi, containing bits [1+i:32+i] of b
     34 
     35 	shrl		%cl,			%eax	// Practically, this means that bhi is given by:
     36 	shrl		%eax					//
     37 	notl		%ecx					//		bhi = (high word of b) << (31 - i) |
     38 	shll		%cl,			%ebx	//			  (low word of b) >> (1 + i)
     39 	orl			%eax,			%ebx	//
     40 	movl	 12(%esp),			%edx	// Load the high and low words of a, and jump
     41 	movl	  8(%esp),			%eax	// to [2] if the high word is larger than bhi
     42 	cmpl		%ebx,			%edx	// to avoid overflowing the upcoming divide.
     43 	jae			2f
     44 
     45 	/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
     46 
     47 	divl		%ebx					// eax <-- qs, edx <-- r such that ahi:alo = bs*qs + r
     48 
     49 	pushl		%edi
     50 	notl		%ecx
     51 	shrl		%eax
     52 	shrl		%cl,			%eax	// q = qs >> (1 + i)
     53 	movl		%eax,			%edi
     54 	mull	 20(%esp)					// q*blo
     55 	movl	 12(%esp),			%ebx
     56 	movl	 16(%esp),			%ecx	// ECX:EBX = a
     57 	subl		%eax,			%ebx
     58 	sbbl		%edx,			%ecx	// ECX:EBX = a - q*blo
     59 	movl	 24(%esp),			%eax
     60 	imull		%edi,			%eax	// q*bhi
     61 	subl		%eax,			%ecx	// ECX:EBX = a - q*b
     62 
     63 	jnc			1f						// if positive, this is the result.
     64 	addl	 20(%esp),			%ebx	// otherwise
     65 	adcl	 24(%esp),			%ecx	// ECX:EBX = a - (q-1)*b = result
     66 1:	movl		%ebx,			%eax
     67 	movl		%ecx,			%edx
     68 
     69 	popl		%edi
     70 	popl		%ebx
     71 	retl
     72 
     73 
     74 2:	/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
     75 
     76 	subl		%ebx,			%edx	// subtract bhi from ahi so that divide will not
     77 	divl		%ebx					// overflow, and find q and r such that
     78 										//
     79 										//		ahi:alo = (1:q)*bhi + r
     80 										//
     81 										// Note that q is a number in (31-i).(1+i)
     82 										// fix point.
     83 
     84 	pushl		%edi
     85 	notl		%ecx
     86 	shrl		%eax
     87 	orl			$0x80000000,	%eax
     88 	shrl		%cl,			%eax	// q = (1:qs) >> (1 + i)
     89 	movl		%eax,			%edi
     90 	mull	 20(%esp)					// q*blo
     91 	movl	 12(%esp),			%ebx
     92 	movl	 16(%esp),			%ecx	// ECX:EBX = a
     93 	subl		%eax,			%ebx
     94 	sbbl		%edx,			%ecx	// ECX:EBX = a - q*blo
     95 	movl	 24(%esp),			%eax
     96 	imull		%edi,			%eax	// q*bhi
     97 	subl		%eax,			%ecx	// ECX:EBX = a - q*b
     98 
     99 	jnc			3f						// if positive, this is the result.
    100 	addl	 20(%esp),			%ebx	// otherwise
    101 	adcl	 24(%esp),			%ecx	// ECX:EBX = a - (q-1)*b = result
    102 3:	movl		%ebx,			%eax
    103 	movl		%ecx,			%edx
    104 
    105 	popl		%edi
    106 	popl		%ebx
    107 	retl
    108 
    109 
    110 
    111 9:	/* High word of b is zero on this branch */
    112 
    113 	movl	 12(%esp),			%eax	// Find qhi and rhi such that
    114 	movl	 16(%esp),			%ecx	//
    115 	xorl		%edx,			%edx	//		ahi = qhi*b + rhi	with	0  rhi < b
    116 	divl		%ecx					//
    117 	movl		%eax,			%ebx	//
    118 	movl	  8(%esp),			%eax	// Find rlo such that
    119 	divl		%ecx					//
    120 	movl		%edx,			%eax	//		rhi:alo = qlo*b + rlo  with 0  rlo < b
    121 	popl		%ebx					//
    122 	xorl		%edx,			%edx	// and return 0:rlo
    123 	retl								//
    124 END_COMPILERRT_FUNCTION(__umoddi3)
    125 
    126 #endif // __i386__
    127