[llvm] [AMDGPU] Account for existing SDWA selections (PR #123221)

[Frederik Harwath via llvm-commits]

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@@ -102,12 +105,47 @@ class SDWAOperand {
   virtual MachineInstr *potentialToConvert(const SIInstrInfo *TII,
                                            const GCNSubtarget &ST,
                                            SDWAOperandsMap *PotentialMatches = nullptr) = 0;
-  virtual bool convertToSDWA(MachineInstr &MI, const SIInstrInfo *TII) = 0;
+  virtual bool convertToSDWA(MachineInstr &MI, const SIInstrInfo *TII,
+                             bool CombineSelections = false) = 0;
 
   MachineOperand *getTargetOperand() const { return Target; }
   MachineOperand *getReplacedOperand() const { return Replaced; }
   MachineInstr *getParentInst() const { return Target->getParent(); }
 
+  /// Fold a \p FoldedOp SDWA selection into an \p ExistingOp existing SDWA
+  /// selection. If the selections are compatible, return the combined
+  /// selection, otherwise return a nullopt. For example, if we have existing
+  /// BYTE_0 Sel and are attempting to fold WORD_1 Sel:
+  ///     BYTE_0 Sel (WORD_1 Sel (%X)) -> BYTE_2 Sel (%X)
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frederik-h wrote:

> So this patch is implementing folding of SDWA into SDWA?

I would not put it this way. The pass does already handle existing SDWA instructions, see e.g. https://github.com/llvm/llvm-project/blob/48d0ef1a07993139e1acf65910704255443103a5/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp#L1032.  It does not account for the selections correctly. This PR changes the handling to skip the creation of a new SDWA instruction and to combine the selections. It's more of a bugfix and not a new feature.

> In general that seems like something we should avoid the need for up-front.

But does this mean that any handling of pre-existing SDWA instructions should be removed from [here](https://github.com/llvm/llvm-project/blob/48d0ef1a07993139e1acf65910704255443103a5/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp#L1032), e.g. by inserting a `assert (!TII->isSDWA(Opcode))`  (or using a more graceful exit)? 

> In terms of avoiding miscompiles, can we do a simpler patch which just avoids the miscompile by skipping SDWA+SDWA cases first?

I will consider this and discuss it with @jrbyrnes, but my impression is that SIPeepholeSDWA should be enabled to handle this case correctly. 

https://github.com/llvm/llvm-project/pull/123221