cortexa57_extra_costs' alu.shift_reg

Andrew Pinski
Sat Jan 11 10:02:00 GMT 2020

  I was looking into reassoc (for PR 93131) and I noticed that the
alu.shift_reg is set to COSTS_N_INSNS (1).  This prevents an
optimization where we combine some if statements into shifts.  I
looked into the Corext A57 software optimization guide[1] and saw that
shift with a register has a latency of 1 which is the same as a shift
with an immediate.  I looked in the history but it was there to begin
with. Is there a reason for this and would it be acceptable if we
change a57's costs to be correct?

I looked into A65, A72, A75, A76, A77 and N1 software optimization
guides too and they have a similar thing of a latency of one cycle and
those are the ones which use the cortexa57_extra_costs variable too.
Note I could not find the A73 guide though.

Andrew Pinski


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