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+ NetWinder Floating Point Emulator
+ (c) Rebel.COM, 1998
+ (c) 1998, 1999 Philip Blundell
+ Direct questions, comments to Scott Bambrough <scottb@netwinder.org>
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ GNU General Public License for more details.
+ You should have received a copy of the GNU General Public License
+ along with this program; if not, write to the Free Software
+ Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+/* This is the kernel's entry point into the floating point emulator.
+It is called from the kernel with code similar to this:
+ sub r4, r5, #4
+ ldrt r0, [r4] @ r0 = instruction
+ adrsvc al, r9, ret_from_exception @ r9 = normal FP return
+ adrsvc al, lr, fpundefinstr @ lr = undefined instr return
+ get_current_task r10
+ mov r8, #1
+ strb r8, [r10, #TSK_USED_MATH] @ set current->used_math
+ add r10, r10, #TSS_FPESAVE @ r10 = workspace
+ ldr r4, .LC2
+ ldr pc, [r4] @ Call FP emulator entry point
+The kernel expects the emulator to return via one of two possible
+points of return it passes to the emulator. The emulator, if
+successful in its emulation, jumps to ret_from_exception (passed in
+r9) and the kernel takes care of returning control from the trap to
+the user code. If the emulator is unable to emulate the instruction,
+it returns via _fpundefinstr (passed via lr) and the kernel halts the
+user program with a core dump.
+On entry to the emulator r10 points to an area of private FP workspace
+reserved in the thread structure for this process. This is where the
+emulator saves its registers across calls. The first word of this area
+is used as a flag to detect the first time a process uses floating point,
+so that the emulator startup cost can be avoided for tasks that don't
+want it.
+This routine does three things:
+1) The kernel has created a struct pt_regs on the stack and saved the
+user registers into it. See /usr/include/asm/proc/ptrace.h for details.
+2) It calls EmulateAll to emulate a floating point instruction.
+EmulateAll returns 1 if the emulation was successful, or 0 if not.
+3) If an instruction has been emulated successfully, it looks ahead at
+the next instruction. If it is a floating point instruction, it
+executes the instruction, without returning to user space. In this
+way it repeatedly looks ahead and executes floating point instructions
+until it encounters a non floating point instruction, at which time it
+returns via _fpreturn.
+This is done to reduce the effect of the trap overhead on each
+floating point instructions. GCC attempts to group floating point
+instructions to allow the emulator to spread the cost of the trap over
+several floating point instructions. */
+ .globl nwfpe_enter
+ mov r4, lr @ save the failure-return addresses
+ mov sl, sp @ we access the registers via 'sl'
+ ldr r5, [sp, #60] @ get contents of PC;
+ bl EmulateAll @ emulate the instruction
+ cmp r0, #0 @ was emulation successful
+ moveq pc, r4 @ no, return failure
+.Lx1: ldrt r6, [r5], #4 @ get the next instruction and
+ @ increment PC
+ and r2, r6, #0x0F000000 @ test for FP insns
+ teq r2, #0x0C000000
+ teqne r2, #0x0D000000
+ teqne r2, #0x0E000000
+ movne pc, r9 @ return ok if not a fp insn
+ str r5, [sp, #60] @ update PC copy in regs
+ mov r0, r6 @ save a copy
+ ldr r1, [sp, #64] @ fetch the condition codes
+ bl checkCondition @ check the condition
+ cmp r0, #0 @ r0 = 0 ==> condition failed
+ @ if condition code failed to match, next insn
+ beq next @ get the next instruction;
+ mov r0, r6 @ prepare for EmulateAll()
+ b emulate @ if r0 != 0, goto EmulateAll
+ @ We need to be prepared for the instructions at .Lx1 and .Lx2
+ @ to fault. Emit the appropriate exception gunk to fix things up.
+ @ ??? For some reason, faults can happen at .Lx2 even with a
+ @ plain LDR instruction. Weird, but it seems harmless.
+ .section .fixup,"ax"
+ .align 2
+.Lfix: mov pc, r9 @ let the user eat segfaults
+ .previous
+ .section __ex_table,"a"
+ .align 3
+ .long .Lx1, .Lfix
+ .previous