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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"
// di_int __divdi3(di_int a, di_int b);
// result = a / b.
// both inputs and the output are 64-bit signed 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(__divdi3)
/* This is currently implemented by wrapping the unsigned divide up in an absolute
value, then restoring the correct sign at the end of the computation. This could
certainly be improved upon. */
pushl %esi
movl 20(%esp), %edx // high word of b
movl 16(%esp), %eax // low word of b
movl %edx, %ecx
sarl $31, %ecx // (b < 0) ? -1 : 0
xorl %ecx, %eax
xorl %ecx, %edx // EDX:EAX = (b < 0) ? not(b) : b
subl %ecx, %eax
sbbl %ecx, %edx // EDX:EAX = abs(b)
movl %edx, 20(%esp)
movl %eax, 16(%esp) // store abs(b) back to stack
movl %ecx, %esi // set aside sign of b
movl 12(%esp), %edx // high word of b
movl 8(%esp), %eax // low word of b
movl %edx, %ecx
sarl $31, %ecx // (a < 0) ? -1 : 0
xorl %ecx, %eax
xorl %ecx, %edx // EDX:EAX = (a < 0) ? not(a) : a
subl %ecx, %eax
sbbl %ecx, %edx // EDX:EAX = abs(a)
movl %edx, 12(%esp)
movl %eax, 8(%esp) // store abs(a) back to stack
xorl %ecx, %esi // sign of result = (sign of a) ^ (sign of b)
pushl %ebx
movl 24(%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 20(%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 16(%esp), %edx // Load the high and low words of a, and jump
movl 12(%esp), %eax // to [1] if the high word is larger than bhi
cmpl %ebx, %edx // to avoid overflowing the upcoming divide.
jae 1f
/* 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 24(%esp) // q*blo
movl 16(%esp), %ebx
movl 20(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 28(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
sbbl $0, %edi // decrement q if remainder is negative
xorl %edx, %edx
movl %edi, %eax
addl %esi, %eax // Restore correct sign to result
adcl %esi, %edx
xorl %esi, %eax
xorl %esi, %edx
popl %edi // Restore callee-save registers
popl %ebx
popl %esi
retl // Return
1: /* 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 24(%esp) // q*blo
movl 16(%esp), %ebx
movl 20(%esp), %ecx // ECX:EBX = a
subl %eax, %ebx
sbbl %edx, %ecx // ECX:EBX = a - q*blo
movl 28(%esp), %eax
imull %edi, %eax // q*bhi
subl %eax, %ecx // ECX:EBX = a - q*b
sbbl $0, %edi // decrement q if remainder is negative
xorl %edx, %edx
movl %edi, %eax
addl %esi, %eax // Restore correct sign to result
adcl %esi, %edx
xorl %esi, %eax
xorl %esi, %edx
popl %edi // Restore callee-save registers
popl %ebx
popl %esi
retl // Return
9: /* High word of b is zero on this branch */
movl 16(%esp), %eax // Find qhi and rhi such that
movl 20(%esp), %ecx //
xorl %edx, %edx // ahi = qhi*b + rhi with 0 ≤ rhi < b
divl %ecx //
movl %eax, %ebx //
movl 12(%esp), %eax // Find qlo such that
divl %ecx //
movl %ebx, %edx // rhi:alo = qlo*b + rlo with 0 ≤ rlo < b
addl %esi, %eax // Restore correct sign to result
adcl %esi, %edx
xorl %esi, %eax
xorl %esi, %edx
popl %ebx // Restore callee-save registers
popl %esi
retl // Return
END_COMPILERRT_FUNCTION(__divdi3)
#endif // __i386__
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