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Diffstat (limited to 'lib/i386/divdi3.s')
| -rw-r--r-- | lib/i386/divdi3.s | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/lib/i386/divdi3.s b/lib/i386/divdi3.s new file mode 100644 index 000000000..3b50266d7 --- /dev/null +++ b/lib/i386/divdi3.s @@ -0,0 +1,160 @@ +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. + +// 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 +.align 4 +.globl ___divdi3 +___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 + +#endif // __i386__ |