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.
