(original) (raw)

One thing I thought about doing a while back and never really wrote a POC for is the following:
- Make FirstCSRCost a property of the MachineBasicBlock (or create a map of MBB\* -> FirstCSRCost)
- Implement a pre-RA pass that will populate the map as follows:
- Identify all blocks with calls
- Find the nearest common dominator (NCD) to all those blocks (not strict so that a block with a call might be that NCD)
- If the NCD is the entry block, CSR allocation is cheap in all blocks
- Make CSR allocation cheap in blocks that are in the dominator tree rooted at NCD

The idea would be to favour CSR allocation in blocks that might be eligible for the prologue and favour splitting in blocks that we'd prefer not to have a prologue in (or before).
Then a CFG such as this:
A
/ \\
B C
| / \\
| D E
| | /
| | /
| |/
| /
|/
F

- Assume calls are in B and any\_of(C,D,E): CSR allocation is cheap everywhere
- Assume calls are in C or all\_of(D,E): CSR allocation is cheap in all\_of(C,D,E); CSR allocation is expensive in all\_of(A,B,F)
- Assume only call is in any\_of(B,D,E): CSR allocation is cheap only in that block, expensive everywhere else


I think this construction would give us what we want, but there may be \[obvious\] counter-examples I haven't thought of.

On Tue, Oct 31, 2017 at 8:38 PM, via llvm-dev <llvm-dev@lists.llvm.org> wrote:
On 2017-10-30 21:20, Hal Finkel wrote:
On 10/30/2017 12:20 PM, junbuml@codeaurora.org wrote:
On 2017-10-27 19:50, Hal Finkel wrote:
On 10/27/2017 03:32 PM, Jun Lim via llvm-dev wrote:

When compiling C code below for AArach64, I saw that shrink-wrapping
didn't happen due to the very early uses of CSRs in the entry block.
So CSR spills/reloads are executed even when the early exit block is
taken.

int getI(int i);

int foo(int \*P, int i) {

if (i>0)

return P\[i\];

i = getI(i);

return P\[i\];

}

It's not that hard to find such cases where RegAllocGreedy
aggressively allocates a CSRs when a live range expands across a
call-site. That's because of the conservatively initialized
CSRCost, causing RegAllocGreedy to strongly favour allocating a CSR
over splitting a region. Since allocation of CSRs requires the cost
of spilling CSRs, allocating CSRs is not always beneficial. Like the
case above, if a function has an early exit code, we may want to be
less aggressive on the first allocation of CSR in the entry block by
increasing the CSRCost.

Previously, I proposed https://reviews.llvm.org/D34608 in this
matter, but the way I detect the profitable cases and the way I
increase the CRSCost was somewhat unclear. Now, I'm thinking to less
aggressive on the first allocation of CSR in the entry block in case
where the function has an early exit so that encourage more
shrink-wrapping and avoid executing CSR spill/recover when the early
exit is taken. By sending this out, I just want to get any high
level feedback early. Please let me know if anyone has any opinion
about this.

So the heuristic will have nothing to do with the presence of calls?
Might this increase spilling in loops?

\-Hal


Before allowing the first allocation from CSRs, I will check if the virtual register is really live across a call in other blocks. If the function have a call in entry or exit, we don't need to increase the CSRCost. This heuristic will be applied only for the very first allocation from CSRs only in the entry block when the function has an early exit; if a CSR is already allocated in the function, we will use the current default global CSRCost.

Even after increasing the CSRCost, we should end up allocating the first CSR if the cost of splitting the live-range is still higher than the increased CSRCost. I believe the amount we want to increase the CSRCost must be target-dependent, but it must be conservative enough to avoid too many copies in the related spots.

Thanks for explaining.

I suppose you'll want to make sure that the call(s) in question come
after the early exit (i.e., that there aren't calls before the early
exit)?


I will check if a virtual register is live across only calls which will be executed when the early exit is not taken. By increasing CSRcost for such case, we increase chances to avoid executing CSR spill/recover when the early exit is taken.

When you say "only in the entry block" you mean that the live range
starts in the entry block, right (i.e., that it is a function
parameter or a vreg defined by some instruction in the entry block)?

 Yes.

Does it matter that it is in the entry block, or do you only need it
to come before the early exit and have an execution frequency <= to
the entry block's execution frequency?

 The reason I specifically check the entry block is because for now I see the early exit happen only in entry block; limiting that the early exit condition is checked in the entry block and branch to the exit block directly from the entry block if hitting the condition. If overall approach is reasonable, we can certainly extend it.


\-Hal


Thanks,
Jun


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\-- Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory


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