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// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
#include "../assembly.h"
// du_int __umoddi3(du_int a, du_int b);
// result = remainder of a / b.
// both inputs and the output are 64-bit unsigned integers.
// This will do whatever the underlying hardware is set to do on division by zero.
// No other exceptions are generated, as the divide cannot overflow.
//
// This is targeted at 32-bit x86 *only*, as this can be done directly in hardware
// on x86_64. The performance goal is ~40 cycles per divide, which is faster than
// currently possible via simulation of integer divides on the x87 unit.
//
// Stephen Canon, December 2008
#ifdef __i386__
.text
.balign 4
DEFINE_COMPILERRT_FUNCTION(__umoddi3)
pushl %ebx
movl 20(%esp), %ebx // Find the index i of the leading bit in b.
bsrl %ebx, %ecx // If the high word of b is zero, jump to
jz 9f // the code to handle that special case [9].
/* High word of b is known to be non-zero on this branch */
movl 16(%esp), %eax // Construct bhi, containing bits [1+i:32+i] of b
shrl %cl, %eax // Practically, this means that bhi is given by:
shrl %eax //
notl %ecx // bhi = (high word of b) << (31 - i) |
shll %cl, %ebx // (low word of b) >> (1 + i)
orl %eax, %ebx //
movl 12(%esp), %edx // Load the high and low words of a, and jump
movl 8(%esp), %eax // to [2] if the high word is larger than bhi
cmpl %ebx, %edx // to avoid overflowing the upcoming divide.
jae 2f
/* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
divl %ebx // eax <-- qs, edx <-- r such that ahi:alo = bs*qs + r
pushl %edi
notl %ecx
shrl %eax
shrl %cl, %eax // q = qs >> (1 + i)
movl %eax, %edi
mull 20(%esp) // q*blo
movl 12(%esp), %ebx
movl 16(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 24(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
jnc 1f // if positive, this is the result.
addl 20(%esp), %ebx // otherwise
adcl 24(%esp), %ecx // ECX:EBX = a - (q-1)*b = result
1: movl %ebx, %eax
movl %ecx, %edx
popl %edi
popl %ebx
retl
2: /* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
subl %ebx, %edx // subtract bhi from ahi so that divide will not
divl %ebx // overflow, and find q and r such that
//
// ahi:alo = (1:q)*bhi + r
//
// Note that q is a number in (31-i).(1+i)
// fix point.
pushl %edi
notl %ecx
shrl %eax
orl $0x80000000, %eax
shrl %cl, %eax // q = (1:qs) >> (1 + i)
movl %eax, %edi
mull 20(%esp) // q*blo
movl 12(%esp), %ebx
movl 16(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 24(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
jnc 3f // if positive, this is the result.
addl 20(%esp), %ebx // otherwise
adcl 24(%esp), %ecx // ECX:EBX = a - (q-1)*b = result
3: movl %ebx, %eax
movl %ecx, %edx
popl %edi
popl %ebx
retl
9: /* High word of b is zero on this branch */
movl 12(%esp), %eax // Find qhi and rhi such that
movl 16(%esp), %ecx //
xorl %edx, %edx // ahi = qhi*b + rhi with 0 ≤ rhi < b
divl %ecx //
movl %eax, %ebx //
movl 8(%esp), %eax // Find rlo such that
divl %ecx //
movl %edx, %eax // rhi:alo = qlo*b + rlo with 0 ≤ rlo < b
popl %ebx //
xorl %edx, %edx // and return 0:rlo
retl //
END_COMPILERRT_FUNCTION(__umoddi3)
#endif // __i386__
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