Re: riscv64 dep. computation

[Jim Wilson <jimw@sifive.com>]

On 2/14/19 3:13 AM, Paulo Matos wrote:
> If I compile this with -O2, sched1 groups all loads and all stores
> together. That's perfect. However, if I change TYPE to unsigned char and
> recompile, the stores and loads are interleaved.
> 
> Further investigation shows that for unsigned char there are extra
> dependencies that block the scheduler from grouping stores and loads.

The ISO C standard says that anything can be casted to char *, and char 
* can be casted to anything.  Hence, a char * pointer aliases everything.

If you look at the alias set info in the MEMs, you can see that the char 
* references are in alias set 0, which means that they alias everything. 
  The short * references are in alias set 2 which means they only alias 
other stuff in alias set 2.  The difference here is that short * does 
not alias the structure pointers, but char * does.  I haven't tried 
debugging your example, but this is presumably where the difference 
comes from.

Because x and y are pointer parameters, the compiler must assume that 
they might alias.  And because char * aliases everything, the char 
references alias them too.  If you change x and y to global variables, 
then they no longer alias each other, and the compiler will schedule all 
of the loads first, even for char.

Jim