f0a290faf8
BUG 1 - choosing the right cycle when booking a resource.
---------------------------------------------------------
Bottom up scheduling should take in account the current cycle at
the scheduling boundary when determing at what cycle a resource can be
issued. Supposed the schedule boundary is at cycle `C`, and that we
want to check at what cycle a 3 cycles resource can be instantiated.
We have two cases: A, in which the last seen resource cycle LSRC in
which the resource is known to be used is more than oe euqual to 3
cycles away from current cycle `C`, (`C - LSRC >=3`) and B in which
the LSRC is less than 3 cycles away from C (`C - LSRC < 3`). Note
that, in bottom-up scheduling LRS is always smaller or eaual to the
current cycle `C`.
The two cases can be schematized as follow:
```
... | C + 1 | C | C - 1 | C - 2 | C - 3 | C - 4 | ...
| | | | | | LSRC | -> Case A
| | | | LSRC | | | -> Case B
// Before allocating the resource
LSRC(A) = C - 4
LSRC(B) = C - 2
```
In case A, the scheduler sees cycles `C`, `C-1` and `C-2` being
available for booking the 3-cycles resource. Therefore the LSRC can be
updated to be `C`, and the resource can be scheduled from cycle `C`
(the `X` in the table):
```
... | C + 1 | C | C - 1 | C - 2 | C - 3 | C - 4 | ...
| | X | X | X | | | -> Case A
// After allocating the resource
LSRC(A) = C
```
In case B, the 3-cycle resource usage would clash with the LSRC if
allocated starting from cycle C:
```
... | C + 1 | C | C - 1 | C - 2 | C - 3 | C - 4 | ...
| | X | X | X | | | -> clash at cycle C - 2
| | | | LSRC | | | -> Case B
```
Therefore, the cycle in which the resource can be scheduled needs to
be greater than `C`. For the example, the resource is booked
in cycle `C + 1`.
```
... | C + 1 | C | C - 1 | C - 2 | C - 3 | C - 4 | ...
| X | X | X | | | |
// After allocating the resource
LSRC(B) = C + 1
```
The behavior we need to correctly support cases A and B is obtained by
computing the next value of the LSRC as the maximum between:
1. the current cycle `C`;
2. and the previous LSRC plus the number of cycle CYCLES the resource will need.
In formula:
```
LSRC(next) = max(C, LSRC(previous) + CYCLES)
```
BUG 2 - booking the resource for the correct number of cycles.
--------------------------------------------------------------
When storing the next LSRC, the funcion `getNextResourceCycle` was
being invoked setting to 0 the number of cycles a resource was using.
The invocation of `getNextResourceCycle` is now using the values of
`Cycles` instead of 0.
Effects on code generation
--------------------------
This fix have effects only on AArch64, for the Cortex-A55
scheduling model (`-mcpu=cortex-a55`).
The changes in the MIR tests caused by this patch show that the value
now reported by `getNextResourceCycle` is correct.
Other cortex-a55 tests have been touched by this change, where some
instructions have been swapped. The final generated code is equivalent
in term of the total number of cycles. The test
`llvm/test/CodeGen/AArch64/misched-detail-resource-booking-02.mir`
shows in details the correctness of the bottom up scheduling, and the
effect on the codegen change that are visible in the test
`llvm/test/CodeGen/AArch64/aarch64-smull.ll`.
Reviewed By: andreadb, dmgreen
Differential Revision: https://reviews.llvm.org/D153117
158 KiB
158 KiB