https://github.com/akkartik/mu/blob/master/subx/058read.subx
  1 # read: analogously to write, support reading from in-memory streams in
  2 # addition to file descriptors.
  3 #
  4 # We can pass it either a file descriptor or an address to a stream. If a
  5 # file descriptor is passed in, we _read from it using the right syscall. If a
  6 # stream is passed in (a fake file descriptor), we read from it instead. This
  7 # lets us initialize input for tests.
  8 #
  9 # A little counter-intuitively, the output of 'read' ends up in.. a stream. So
 10 # tests end up doing a redundant copy. Why? Well, consider the alternatives:
 11 #
 12 #   a) Reading into a string, and returning a pointer to the end of the read
 13 #   region, or a count of bytes written. Now this count or end pointer must be
 14 #   managed separately by the caller, which can be error-prone.
 15 #
 16 #   b) Having 'read' return a buffer that it allocates. But there's no way to
 17 #   know in advance how large to make the buffer. If you read less than the
 18 #   size of the buffer you again end up needing to manage initialized vs
 19 #   uninitialized memory.
 20 #
 21 #   c) Creating more helpful variants like 'read-byte' or 'read-until' which
 22 #   also can take a file descriptor or stream, just like 'write'. But such
 23 #   primitives don't exist in the Linux kernel, so we'd be implementing them
 24 #   somehow, either with more internal buffering or by making multiple
 25 #   syscalls.
 26 #
 27 # Reading into a stream avoids these problems. The buffer is externally
 28 # provided and the caller has control over where it's allocated, its lifetime,
 29 # and so on. The buffer's read and write pointers are internal to it so it's
 30 # easier to keep in a consistent state. And it can now be passed directly to
 31 # helpers like 'read-byte' or 'read-until' that only need to support streams,
 32 # never file descriptors.
 33 #
 34 # Like with 'write', we assume our data segment will never begin at an address
 35 # shorter than 0x08000000, so any smaller arguments are assumed to be real
 36 # file descriptors.
 37 #
 38 # As a reminder, a stream looks like this:
 39 #   write: int  # index at which to write to next
 40 #   read: int  # index at which to read next
 41 #   data: (array byte)  # prefixed by length as usual
 42 
 43 == code
 44 #   instruction                     effective address                                                   register    displacement    immediate
 45 # . op          subop               mod             rm32          base        index         scale       r32
 46 # . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes
 47 
 48 # main:
 49     e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
 50     # syscall(exit, Num-test-failures)
 51     8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
 52     b8/copy-to-EAX  1/imm32
 53     cd/syscall  0x80/imm8
 54 
 55 read:  # f : fd or (address stream), s : (address stream) -> num-bytes-read/EAX
 56     # . prolog
 57     55/push-EBP
 58     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 59     # if (f < 0x08000000) return _read(f, s)  # f can't be a user-mode address, so treat it as a kernel file descriptor
 60     81          7/subop/compare     1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         0x08000000/imm32  # compare *(EBP+8)
 61     7d/jump-if-greater-or-equal  $read:fake/disp8
 62     # . . push args
 63     ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0xc/disp8       .                 # push *(EBP+12)
 64     ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         .                 # push *(EBP+8)
 65     # . . call
 66     e8/call  _read/disp32
 67     # . . discard args
 68     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
 69     # return
 70     eb/jump  $read:end/disp8
 71 $read:fake:
 72     # otherwise, treat 'f' as a stream to scan from
 73     # . save registers
 74     56/push-ESI
 75     57/push-EDI
 76     # ESI = f
 77     8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           6/r32/ESI   8/disp8         .                 # copy *(EBP+8) to ESI
 78     # EDI = s
 79     8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           7/r32/EDI   0xc/disp8       .                 # copy *(EBP+12) to ESI
 80     # EAX = _append-4(out = &s->data[s->write], outend = &s->data[s->length],
 81     #                 in  = &f->data[f->read],  inend  = &f->data[f->write])
 82     # . . push &f->data[f->write]
 83     8b/copy                         0/mod/indirect  6/rm32/ESI    .           .             .           0/r32/EAX   .               .                 # copy *ESI to EAX
 84     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    6/base/ESI  0/index/EAX   .           0/r32/EAX   0xc/disp8       .                 # copy ESI+EAX+12 to EAX
 85     50/push-EAX
 86     # . . push &f->data[f->read]
 87     8b/copy                         1/mod/*+disp8   6/rm32/ESI    .           .             .           0/r32/EAX   4/disp8         .                 # copy *(ESI+4) to EAX
 88     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    6/base/ESI  0/index/EAX   .           0/r32/EAX   0xc/disp8       .                 # copy ESI+EAX+12 to EAX
 89     50/push-EAX
 90     # . . push &s->data[s->length]
 91     8b/copy                         1/mod/*+disp8   7/rm32/EDI    .           .             .           0/r32/EAX   8/disp8         .                 # copy *(EDI+8) to EAX
 92     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/EDI  0/index/EAX   .           0/r32/EAX   0xc/disp8       .                 # copy EDI+EAX+12 to EAX
 93     50/push-EAX
 94     # . . push &s->data[s->write]
 95     8b/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy *EDI to EAX
 96     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/EDI  0/index/EAX   .           0/r32/EAX   0xc/disp8       .                 # copy EDI+EAX+12 to EAX
 97     50/push-EAX
 98     # . . call
 99     e8/call  _append-4/disp32
100     # . . discard args
101     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0x10/imm32        # add to ESP
102     # s->write += EAX
103     01/add                          0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # add EAX to *EDI
104     # f->read += EAX
105     01/add                          1/mod/*+disp8   6/rm32/ESI    .           .             .           0/r32/EAX   4/disp8         .                 # add EAX to *(ESI+4)
106     # . restore registers
107     5f/pop-to-EDI
108     5e/pop-to-ESI
109 $read:end:
110     # . epilog
111     89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
112     5d/pop-to-EBP
113     c3/return
114 
115 # - helpers
116 
117 # idea: a clear-if-empty method on streams that clears only if f->read == f->write
118 # Unclear how I'd use it, though. Callers seem to need the check anyway.
119 # Maybe a better helper would be 'empty-stream?'
120 
121 _read:  # fd : int, s : (address stream) -> num-bytes-read/EAX
122     # . prolog
123     55/push-EBP
124     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
125     # . save registers
126     51/push-ECX
127     52/push-EDX
128     53/push-EBX
129     56/push-ESI
130     # ESI = s
131     8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
132     # EAX = s->write
133     8b/copy                         0/mod/indirect  6/rm32/ESI    .           .             .           0/r32/EAX   .               .                 # copy *ESI to EAX
134     # EDX = s->length
135     8b/copy                         1/mod/*+disp8   6/rm32/ESI    .           .             .           2/r32/EDX   8/disp8         .                 # copy *(ESI+8) to EDX
136     # syscall(read, fd, &s->data[s->write], s->length - s->write)
137     # . . fd : EBX
138     8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           3/r32/EBX   8/disp8         .                 # copy *(EBP+8) to EBX
139     # . . data : ECX = &s->data[s->write]
140     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    6/base/ESI  0/index/EAX   .           1/r32/ECX   0xc/disp8       .                 # copy ESI+EAX+12 to ECX
141     # . . size : EDX = s->length - s->write
142     29/subtract                     3/mod/direct    2/rm32/EDX    .           .             .           0/r32/EAX   .               .                 # subtract EAX from EDX
143     # . . syscall
144     b8/copy-to-EAX  3/imm32/read
145     cd/syscall  0x80/imm8
146     # add the result EAX to s->write
147     01/add                          0/mod/indirect  6/rm32/ESI    .           .             .           0/r32/EAX   .               .                 # add EAX to *ESI
148 $_read:end:
149     # . restore registers
150     5e/pop-to-ESI
151     5b/pop-to-EBX
152     5a/pop-to-EDX
153     59/pop-to-ECX
154     # . epilog
155     89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
156     5d/pop-to-EBP
157     c3/return
158 
159     # Two options:
160     #   1 (what we have above):
161     #     ECX = s
162     #     EAX = s->write
163     #     EDX = s->length
164     #     # syscall
165     #     ECX = lea ECX+EAX+12
166     #     EDX = sub EDX EAX
167     #
168     #   2:
169     #     ECX = s
170     #     EDX = s->length
171     #     ECX = &s->data
172     #     # syscall
173     #     ECX = add ECX, s->write
174     #     EDX = sub EDX, s->write
175     #
176     # Not much to choose between the two? Option 2 performs a duplicate load to
177     # use one less register, but doesn't increase the amount of spilling (ECX
178     # and EDX must be used, and EAX must be clobbered anyway).
179 
180 # - tests
181 
182 test-read-single:
183     # - write a single character into _test-stream, then read from its buffered-file
184     # clear-stream(_test-stream)
185     # . . push args
186     68/push  _test-stream/imm32
187     # . . call
188     e8/call  clear-stream/disp32
189     # . . discard args
190     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
191     # clear-stream(_test-stream-buffer)
192     # . . push args
193     68/push  _test-stream-buffer/imm32
194     # . . call
195     e8/call  clear-stream/disp32
196     # . . discard args
197     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
198     # write(_test-stream, "Ab")
199     # . . push args
200     68/push  "Ab"/imm32
201     68/push  _test-stream/imm32
202     # . . call
203     e8/call  write/disp32
204     # . . discard args
205     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
206     # read(_test-stream, _test-stream-buffer)
207     # . . push args
208     68/push  _test-stream-buffer/imm32
209     68/push  _test-stream/imm32
210     # . . call
211     e8/call  read/disp32
212     # . . discard args
213     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
214     # check-ints-equal(EAX, 2, msg)
215     # . . push args
216     68/push  "F - test-read-single: return EAX"/imm32
217     68/push  2/imm32
218     50/push-EAX
219     # . . call
220     e8/call  check-ints-equal/disp32
221     # . . discard args
222     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
223     # check-ints-equal(*_test-stream-buffer->data, 41/A 62/b 00 00, msg)
224     # . . push args
225     68/push  "F - test-read-single"/imm32
226     68/push  0x006241/imm32/Ab
227     # . . push *_test-stream-buffer->data
228     b8/copy-to-EAX  _test-stream-buffer/imm32
229     ff          6/subop/push        1/mod/*+disp8   0/rm32/EAX    .           .             .           .           0xc/disp8       .                 # push *(EAX+12)
230     # . . call
231     e8/call  check-ints-equal/disp32
232     # . . discard args
233     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
234     # end
235     c3/return
236 
237 test-read-is-stateful:
238     # - make two consecutive reads, check that their results are appended
239     # clear-stream(_test-stream)
240     # . . push args
241     68/push  _test-stream/imm32
242     # . . call
243     e8/call  clear-stream/disp32
244     # . . discard args
245     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
246     # clear-stream(_test-stream-buffer)
247     # . . push args
248     68/push  _test-stream-buffer/imm32
249     # . . call
250     e8/call  clear-stream/disp32
251     # . . discard args
252     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
253     # write(_test-stream, "C")
254     # . . push args
255     68/push  "C"/imm32
256     68/push  _test-stream/imm32
257     # . . call
258     e8/call  write/disp32
259     # . . discard args
260     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
261     # read(_test-stream, _test-stream-buffer)
262     # . . push args
263     68/push  _test-stream-buffer/imm32
264     68/push  _test-stream/imm32
265     # . . call
266     e8/call  read/disp32
267     # . . discard args
268     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
269     # write(_test-stream, "D")
270     # . . push args
271     68/push  "D"/imm32
272     68/push  _test-stream/imm32
273     # . . call
274     e8/call  write/disp32
275     # . . discard args
276     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
277     # read(_test-stream, _test-stream-buffer)
278     # . . push args
279     68/push  _test-stream-buffer/imm32
280     68/push  _test-stream/imm32
281     # . . call
282     e8/call  read/disp32
283     # . . discard args
284     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
285     # check-ints-equal(*_test-stream-buffer->data, 43/C 44/D 00 00, msg)
286     # . . push args
287     68/push  "F - test-read-is-stateful"/imm32
288     68/push  0x00004443/imm32/C-D
289     # . . push *_test-stream-buffer->data
290     b8/copy-to-EAX  _test-stream-buffer/imm32
291     ff          6/subop/push        1/mod/*+disp8   0/rm32/EAX    .           .             .           .           0xc/disp8       .                 # push *(EAX+12)
292     # . . call
293     e8/call  check-ints-equal/disp32
294     # . . discard args
295     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
296     # end
297     c3/return
298 
299 test-read-returns-0-on-end-of-file:
300     # - read after hitting end-of-file, check that result is 0
301     # setup
302     # . clear-stream(_test-stream)
303     # . . push args
304     68/push  _test-stream/imm32
305     # . . call
306     e8/call  clear-stream/disp32
307     # . . discard args
308     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
309     # . clear-stream(_test-stream-buffer)
310     # . . push args
311     68/push  _test-stream-buffer/imm32
312     # . . call
313     e8/call  clear-stream/disp32
314     # . . discard args
315     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
316     # . write(_test-stream, "Ab")
317     # . . push args
318     68/push  "Ab"/imm32
319     68/push  _test-stream/imm32
320     # . . call
321     e8/call  write/disp32
322     # . . discard args
323     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
324     # first read gets to end-of-file
325     # . read(_test-stream, _test-stream-buffer)
326     # . . push args
327     68/push  _test-stream-buffer/imm32
328     68/push  _test-stream/imm32
329     # . . call
330     e8/call  read/disp32
331     # . . discard args
332     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
333     # second read
334     # . read(_test-stream, _test-stream-buffer)
335     # . . push args
336     68/push  _test-stream-buffer/imm32
337     68/push  _test-stream/imm32
338     # . . call
339     e8/call  read/disp32
340     # . . discard args
341     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
342     # check-ints-equal(EAX, 0, msg)
343     # . . push args
344     68/push  "F - test-read-returns-0-on-end-of-file"/imm32
345     68/push  0/imm32
346     50/push-EAX
347     # . . call
348     e8/call  check-ints-equal/disp32
349     # . . discard args
350     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
351     # end
352     c3/return
353 
354 == data
355 
356 _test-stream-buffer:
357     # current write index
358     00 00 00 00
359     # current read index
360     00 00 00 00
361     # length (= 8)
362     08 00 00 00
363     # data
364     00 00 00 00 00 00 00 00  # 8 bytes
365 
366 # . . vim:nowrap:textwidth=0