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This patch changes the location of the frame-record (FP, LR) to the
bottom of the callee-saved area. According to the AAPCS the location of
the frame-record within the stackframe is unspecified (section 5.2.3 The
Frame Pointer), so the compiler should be free to choose a different
location.
The reason for changing the location of the frame-record is to prepare
the frame for allocating an SVE area below the callee-saves. This way the
compiler can use the VL-scaled addressing modes to directly access SVE
objects from the frame-pointer.
: :
| stack | | stack |
| args | | args |
+-------+ +-------+
| x30 | | x19 |
| x29 | | x20 |
FP -> |- - - -| | x21 |
| x19 | ==> | x22 |
| x20 | |- - - -|
| x21 | | x30 |
| x22 | | x29 |
+-------+ +-------+ <- FP
|///////| |///////| // realignment gap
|- - - -| |- - - -|
|spills/| |spills/|
| locals| | locals|
SP -> +-------+ +-------+ <- SP
Things to point out:
- The algorithm to find a paired register should be prevented from
accidentally pairing some callee-saved register with LR that is not
FP, since they should always be paired together when the frame
has a frame-record.
- For Darwin platforms the location of the frame-record is unchanged,
since the unwind encoding does not allow for encoding this position
dynamically and other tools currently depend on the former layout.
Reviewers: efriedma, rovka, rengolin, thegameg, greened, t.p.northover
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D65653
llvm-svn: 368987
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Summary:
This pass sinks COPY instructions into a successor block, if the COPY is not
used in the current block and the COPY is live-in to a single successor
(i.e., doesn't require the COPY to be duplicated). This avoids executing the
the copy on paths where their results aren't needed. This also exposes
additional opportunites for dead copy elimination and shrink wrapping.
These copies were either not handled by or are inserted after the MachineSink
pass. As an example of the former case, the MachineSink pass cannot sink
COPY instructions with allocatable source registers; for AArch64 these type
of copy instructions are frequently used to move function parameters (PhyReg)
into virtual registers in the entry block..
For the machine IR below, this pass will sink %w19 in the entry into its
successor (%bb.1) because %w19 is only live-in in %bb.1.
```
%bb.0:
%wzr = SUBSWri %w1, 1
%w19 = COPY %w0
Bcc 11, %bb.2
%bb.1:
Live Ins: %w19
BL @fun
%w0 = ADDWrr %w0, %w19
RET %w0
%bb.2:
%w0 = COPY %wzr
RET %w0
```
As we sink %w19 (CSR in AArch64) into %bb.1, the shrink-wrapping pass will be
able to see %bb.0 as a candidate.
With this change I observed 12% more shrink-wrapping candidate and 13% more dead copies deleted in spec2000/2006/2017 on AArch64.
Reviewers: qcolombet, MatzeB, thegameg, mcrosier, gberry, hfinkel, john.brawn, twoh, RKSimon, sebpop, kparzysz
Reviewed By: sebpop
Subscribers: evandro, sebpop, sfertile, aemerson, mgorny, javed.absar, kristof.beyls, llvm-commits
Differential Revision: https://reviews.llvm.org/D41463
llvm-svn: 328237
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