[Bug target/95704] New: PPC: int128 shifts should be implemented branchless

[jens.seifert at de dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95704

            Bug ID: 95704
           Summary: PPC: int128 shifts should be implemented branchless
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jens.seifert at de dot ibm.com
  Target Milestone: ---

Created attachment 48741
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48741&action=edit
input with branchless 128-bit shifts

PowerPC processors don't like branches and branch mispredicts lead to large
overhead.

shift left/right unsigned __in128 can be implemented in 8 instructions which
can be processed on 2 pipelines almost in parallel leading to ~5 cycle latency
on Power 7 and 8.
shift right algebraic __int128 can be implemented in 10 instructions.
Overall comparable in latency of the branching code.

In attached file you find the branch less implementations in C. And I know that
this is using undefined behavior. But the resulting assembly is the interesting
part. 

The unnecessary rldicl 8,5,0,32 at the beginning of the routines are also not
necessary.