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1/*
2 NetWinder Floating Point Emulator
3 (c) Rebel.COM, 1998
4 (c) 1998, 1999 Philip Blundell
5
6 Direct questions, comments to Scott Bambrough <scottb@netwinder.org>
7
8 This program is free software; you can redistribute it and/or modify
9 it under the terms of the GNU General Public License as published by
10 the Free Software Foundation; either version 2 of the License, or
11 (at your option) any later version.
12
13 This program is distributed in the hope that it will be useful,
14 but WITHOUT ANY WARRANTY; without even the implied warranty of
15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 GNU General Public License for more details.
17
18 You should have received a copy of the GNU General Public License
19 along with this program; if not, write to the Free Software
20 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
21*/
22
23/* This is the kernel's entry point into the floating point emulator.
24It is called from the kernel with code similar to this:
25
26 sub r4, r5, #4
27 ldrt r0, [r4] @ r0 = instruction
28 adrsvc al, r9, ret_from_exception @ r9 = normal FP return
29 adrsvc al, lr, fpundefinstr @ lr = undefined instr return
30
31 get_current_task r10
32 mov r8, #1
33 strb r8, [r10, #TSK_USED_MATH] @ set current->used_math
34 add r10, r10, #TSS_FPESAVE @ r10 = workspace
35 ldr r4, .LC2
36 ldr pc, [r4] @ Call FP emulator entry point
37
38The kernel expects the emulator to return via one of two possible
39points of return it passes to the emulator. The emulator, if
40successful in its emulation, jumps to ret_from_exception (passed in
41r9) and the kernel takes care of returning control from the trap to
42the user code. If the emulator is unable to emulate the instruction,
43it returns via _fpundefinstr (passed via lr) and the kernel halts the
44user program with a core dump.
45
46On entry to the emulator r10 points to an area of private FP workspace
47reserved in the thread structure for this process. This is where the
48emulator saves its registers across calls. The first word of this area
49is used as a flag to detect the first time a process uses floating point,
50so that the emulator startup cost can be avoided for tasks that don't
51want it.
52
53This routine does three things:
54
551) The kernel has created a struct pt_regs on the stack and saved the
56user registers into it. See /usr/include/asm/proc/ptrace.h for details.
57
582) It calls EmulateAll to emulate a floating point instruction.
59EmulateAll returns 1 if the emulation was successful, or 0 if not.
60
613) If an instruction has been emulated successfully, it looks ahead at
62the next instruction. If it is a floating point instruction, it
63executes the instruction, without returning to user space. In this
64way it repeatedly looks ahead and executes floating point instructions
65until it encounters a non floating point instruction, at which time it
66returns via _fpreturn.
67
68This is done to reduce the effect of the trap overhead on each
69floating point instructions. GCC attempts to group floating point
70instructions to allow the emulator to spread the cost of the trap over
71several floating point instructions. */
72
73 .globl nwfpe_enter
74nwfpe_enter:
75 mov r4, lr @ save the failure-return addresses
76 mov sl, sp @ we access the registers via 'sl'
77
78 ldr r5, [sp, #60] @ get contents of PC;
79emulate:
80 bl EmulateAll @ emulate the instruction
81 cmp r0, #0 @ was emulation successful
82 moveq pc, r4 @ no, return failure
83
84next:
85.Lx1: ldrt r6, [r5], #4 @ get the next instruction and
86 @ increment PC
87
88 and r2, r6, #0x0F000000 @ test for FP insns
89 teq r2, #0x0C000000
90 teqne r2, #0x0D000000
91 teqne r2, #0x0E000000
92 movne pc, r9 @ return ok if not a fp insn
93
94 str r5, [sp, #60] @ update PC copy in regs
95
96 mov r0, r6 @ save a copy
97 ldr r1, [sp, #64] @ fetch the condition codes
98 bl checkCondition @ check the condition
99 cmp r0, #0 @ r0 = 0 ==> condition failed
100
101 @ if condition code failed to match, next insn
102 beq next @ get the next instruction;
103
104 mov r0, r6 @ prepare for EmulateAll()
105 b emulate @ if r0 != 0, goto EmulateAll
106
107 @ We need to be prepared for the instructions at .Lx1 and .Lx2
108 @ to fault. Emit the appropriate exception gunk to fix things up.
109 @ ??? For some reason, faults can happen at .Lx2 even with a
110 @ plain LDR instruction. Weird, but it seems harmless.
111 .section .fixup,"ax"
112 .align 2
113.Lfix: mov pc, r9 @ let the user eat segfaults
114 .previous
115
116 .section __ex_table,"a"
117 .align 3
118 .long .Lx1, .Lfix
119 .previous