[PATCH] D89239: [RISCV][PrologEpilogInserter] "Float" emergency spill slots to avoid making them immediately unreachable from the stack pointer

[Roger Ferrer Ibanez via Phabricator via llvm-commits]

rogfer01 added inline comments.


================
Comment at: llvm/test/CodeGen/RISCV/out-of-reach-emergency-slot.mir:1
+# NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+# REQUIRES: asserts
----------------
arichardson wrote:
> rogfer01 wrote:
> > arichardson wrote:
> > > sdesmalen wrote:
> > > > This test seems quite substantial, is there no simpler IR to test this?
> > > This test can definitely be reduced a lot more. Also you could run just this pass and make it a MIR test instead.
> > The test is already an MIR test. The YAML representation contains a copy of the IR (which it is required to create essential data structures such as `llvm::Function`). I can remove most of it as I understand adds none to little information here.
> > 
> > I will look into how to reduce the large amount of machine instructions: part of the difficulty is making sure no register is left for the register scavenger when eliminating the frame index of the problematic stack slot (the one that used to be, before this change, too far from the top of the stack).
> > 
> > I'll look into this again because I think it is possible to do so.
> I meant using -run-pass=prologepilog and using update_mir_checks.py to generate the CHECK lines.
> 
> I just ran creduce over your test case and it gave me this:
> 
> ```
> # RUN: llc -mtriple riscv64 -start-before=prologepilog -o - -verify-machineinstrs %s
> name:            a
> stack:
>   - { id: 0 , size: 8, alignment: 4096 }
> body:             |
>   bb.0:
>     $= ADD $, $x10
>     SD $x12, $, $x13
>     $= DIVU $, $x30
>     $= DIVU $, $x29
>     $= DIV $, $x15
>     $= DIV $, $x14
>     $= DIV $, $x1
>     $= DIV $, $x7
>     $= DIV $, $x6
>     $= DIV $, $x31
>     $= DIV $, $x28
>     $= DIV $, $x5
>     $= DIV $, $x17
>     $= DIV $, $x16
>     $= DIV $x11, $
> ```
> 
> Feeding that into `/bin/llc -mtriple riscv64 -start-before=prologepilog -o - -verify-machineinstrs -stop-before=prologepilog`
>  yields this:
> ```
> --- |
>   ; ModuleID = '/local/scratch/alr48/cheri/build/llvm-project-build/test-reduce.mir'
>   source_filename = "/local/scratch/alr48/cheri/build/llvm-project-build/test-reduce.mir"
>   target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n64-S128"
>   target triple = "riscv64"
> 
>   define void @a() {
>   entry:
>     unreachable
>   }
> 
> ...
> ---
> name:            a
> alignment:       1
> exposesReturnsTwice: false
> legalized:       false
> regBankSelected: false
> selected:        false
> failedISel:      false
> tracksRegLiveness: false
> hasWinCFI:       false
> registers:       []
> liveins:         []
> frameInfo:
>   isFrameAddressTaken: false
>   isReturnAddressTaken: false
>   hasStackMap:     false
>   hasPatchPoint:   false
>   stackSize:       0
>   offsetAdjustment: 0
>   maxAlignment:    4096
>   adjustsStack:    false
>   hasCalls:        false
>   stackProtector:  ''
>   maxCallFrameSize: 4294967295
>   cvBytesOfCalleeSavedRegisters: 0
>   hasOpaqueSPAdjustment: false
>   hasVAStart:      false
>   hasMustTailInVarArgFunc: false
>   localFrameSize:  0
>   savePoint:       ''
>   restorePoint:    ''
> fixedStack:      []
> stack:
>   - { id: 0, name: '', type: default, offset: 0, size: 8, alignment: 4096,
>       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
>       debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
> callSites:       []
> constants:       []
> machineFunctionInfo: {}
> body:             |
>   bb.0:
>     $noreg = ADD $noreg, $x10
>     SD $x12, $noreg, $x13
>     $noreg = DIVU $noreg, $x30
>     $noreg = DIVU $noreg, $x29
>     $noreg = DIV $noreg, $x15
>     $noreg = DIV $noreg, $x14
>     $noreg = DIV $noreg, $x1
>     $noreg = DIV $noreg, $x7
>     $noreg = DIV $noreg, $x6
>     $noreg = DIV $noreg, $x31
>     $noreg = DIV $noreg, $x28
>     $noreg = DIV $noreg, $x5
>     $noreg = DIV $noreg, $x17
>     $noreg = DIV $noreg, $x16
>     $noreg = DIV $x11, $noreg
> 
> ...
> ```
Thanks @arichardson for the reduced testcase.

However, I'm not sure we'd be testing the right thing here. We need a frame index that when eliminated has to use the emergency spill slot. Something like the `SD` instruction in line 500:

```
    SD killed renamable $x12, %stack.1, 0
```

Maybe I'm missing something.


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  https://reviews.llvm.org/D89239/new/

https://reviews.llvm.org/D89239