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Re: riscv64 dep. computation


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


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