Re: Patch for PR target/20632

[Geert Bosch <bosch at adacore dot com>]

On Apr 1, 2005, at 03:06, David Mosberger wrote:
> More curiously, I found a handful of bundles containing only NOPs.
> For example:
>
> 00 00 00 00 01 00       [MII]       nop.m 0x0
> a0 02 ae 3e 29 00                   shr.u r42=r43,32
> 00 00 04 00                         nop.i 0x0
> 0f 00 00 00 01 00       [MMF]       nop.m 0x0
> 00 00 00 02 00 00                   nop.m 0x0
> 00 00 04 00                         nop.f 0x0;;
>
> Is there a good reason to emit such a NOP-only bundle?

This is a direct result of GCC's new tree-ssa optimizers performing
load- and store elimination and advanced strengh-reduction on your
code. In combination with the new DFA scheduler's just-in-time 
scheduling,
which prevents executing any instructions until absolutely necessary,
this allows drastic code simplification.

Instead of executing complex instructions, such a memory loads and
stores that may take many cycles to complete, the processor now only
needs to execute these much simplified instructions. Taking advantage
of the explicit parallelism in the code, the current Itanium-2 
processors
are capable of executing up to 6 of these instructions in one cycle,
thereby fully realizing the promises of Intel's EPIC architecture by
computing absolutely nothing at unprecedented speeds.

  -Geert