On Fri, 28 Jan 2000, Jesse Pollard wrote:
[SNIPPED..]
>
> Just curious, what happens to the time if they are mixed (addl between
> the leal using different registers)? Doesn't the address calculation overlap
> the arithmetic operation? The example looks rather artificial to demonstrate
> the worst case by filling the hardware pipeline without the option of
> re-ordering.
This was not an example of how to do pipe-lining.
>
> >Unfortunately, I see that much of the indexed address generation
> >done in new versions of the C compiler abitrarily use LEA, followed
> >immediately by the memory fetch using the newly generated address.
> >This will be slower than simple math to obtain the address.
>
> Doesn't hat sound more like a lack of proper optimization? If it were
> optimized I would expect to see some addl on index registers mixed with
> leal on other registers.
The recent versions of the GCC compiler output shows that where simple
offsets were previously calculated with early versions, there is now the
excessive use of LEA, with the calculated address immediately being used.
This is as though somebody just changed the code generator to use LEA
wherever possible. Here's an example, the dumped output from sched.o.
I show the multiple times where LEA is used to calculate an address
without an intervening instruction(s) before this address is used.
This is from GCC version 2.7.2.3, the exact utility that
../linux/Documentation/Changes requires.
In most cases LEA is used where an offset is already known at compile-
time. Note the stack operations using LEA and that the "pop" instruction
uses the stack-pointer that has just been changed in value using LEA. This
is a 3-clock penality.
333: 8d 65 e0 lea 0xffffffe0(%ebp),%esp
336: 5b pop %ebx
0000033c <process_timeout>:
667: 8d 65 e0 lea 0xffffffe0(%ebp),%esp
66a: 5b pop %ebx
00000670 <schedule_timeout>:
6da: 8d 5d ec lea 0xffffffec(%ebp),%ebx
6dd: 53 push %ebx
00000708 <schedule_tail>:
9fe: 8d 65 e4 lea 0xffffffe4(%ebp),%esp
a01: 5b pop %ebx
00000a08 <schedule>:
d78: 8d 1c 9d 0d 00 00 00 lea 0xd(,%ebx,4),%ebx
d7f: 8d 0c dd 00 00 00 00 lea 0x0(,%ebx,8),%ecx
dad: 8d 76 00 lea 0x0(%esi),%esi ! NOP
fda: 8d 5c 1e 01 lea 0x1(%esi,%ebx,1),%ebx
fde: 89 5d cc mov %ebx,0xffffffcc(%ebp)
1005: 8d 76 00 lea 0x0(%esi),%esi ! NOP
10a2: 8d 44 02 01 lea 0x1(%edx,%eax,1),%eax
10a6: 89 45 d4 mov %eax,0xffffffd4(%ebp)
1115: 8d 76 00 lea 0x0(%esi),%esi
1118: 8b 71 38 mov 0x38(%ecx),%esi
1335: 8d 76 00 lea 0x0(%esi),%esi ! NOP
146c: 8d 65 c0 lea 0xffffffc0(%ebp),%esp
00001474 <__wake_up>:
19ee: 8d 65 c4 lea 0xffffffc4(%ebp),%esp
19f1: 5b pop %ebx
21a3: 8d 65 e0 lea 0xffffffe0(%ebp),%esp
21a6: 5b pop %ebx
000021ac <interruptible_sleep_on_timeout>:
23e4: 8d 65 dc lea 0xffffffdc(%ebp),%esp
23e7: 5b pop %ebx
000023ec <sleep_on>:
261b: 8d 65 e0 lea 0xffffffe0(%ebp),%esp
261e: 5b pop %ebx
00002624 <sleep_on_timeout>:
2656: 8d 4d f8 lea 0xfffffff8(%ebp),%ecx
2659: 89 4d f8 mov %ecx,0xfffffff8(%ebp)
2745: 8d 4d f0 lea 0xfffffff0(%ebp),%ecx
2748: 89 48 04 mov %ecx,0x4(%eax)
274e: 8d 4b 08 lea 0x8(%ebx),%ecx
2751: 89 4d f4 mov %ecx,0xfffffff4(%ebp)
2754: 8d 4d f0 lea 0xfffffff0(%ebp),%ecx
2757: 89 4b 08 mov %ecx,0x8(%ebx)
285c: 8d 65 dc lea 0xffffffdc(%ebp),%esp
285f: 5b pop %ebx
2b69: 8d 65 e8 lea 0xffffffe8(%ebp),%esp
2b6c: 5b pop %ebx
00002c28 <sys_sched_getparam>:
2d08: 8d 65 ec lea 0xffffffec(%ebp),%esp
2d0b: 5b pop %ebx
00002e30 <sys_sched_rr_get_interval>:
2e81: 8d 65 ec lea 0xffffffec(%ebp),%esp
2e84: 5b pop %ebx
00002e8c <show_task>:
2e98: 8d 87 1a 03 00 00 lea 0x31a(%edi),%eax
2e9e: 50 push %eax
2f24: 8d 90 08 fb ff ff lea 0xfffffb08(%eax),%edx
2f2a: 29 fa sub %edi,%edx
2fe5: 8d 75 ec lea 0xffffffec(%ebp),%esi
2fe8: 56 push %esi
2fe9: 8d 87 ac 04 00 00 lea 0x4ac(%edi),%eax
2fef: 50 push %eax
2ff5: 8d 5d d8 lea 0xffffffd8(%ebp),%ebx
2ff8: 53 push %ebx
2ff9: 8d 87 b4 04 00 00 lea 0x4b4(%edi),%eax
2fff: 50 push %eax
304c: 8d 65 cc lea 0xffffffcc(%ebp),%esp
304f: 5b pop %ebx
00003054 <render_sigset_t>:
3079: 8d 79 02 lea 0x2(%ecx),%edi
307c: 89 7d fc mov %edi,0xfffffffc(%ebp)
30d8: 8d 65 f0 lea 0xfffffff0(%ebp),%esp
30db: 5b pop %ebx
00000000 <init_idle>:
5e: 8d 65 f8 lea 0xfffffff8(%ebp),%esp
62: 5e pop %esi
Cheers,
Dick Johnson
Penguin : Linux version 2.3.39 on an i686 machine (800.63 BogoMips).
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This archive was generated by hypermail 2b29 : Mon Jan 31 2000 - 21:00:21 EST