[llvm] [SLP] Make getSameOpcode support interchangeable instructions. (PR #133888)

[Gaëtan Bossu via llvm-commits]

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@@ -813,6 +835,271 @@ static std::optional<unsigned> getExtractIndex(const Instruction *E) {
 }
 
 namespace {
+/// \returns true if \p Opcode is allowed as part of the main/alternate
+/// instruction for SLP vectorization.
+///
+/// Example of unsupported opcode is SDIV that can potentially cause UB if the
+/// "shuffled out" lane would result in division by zero.
+bool isValidForAlternation(unsigned Opcode) {
+  return !Instruction::isIntDivRem(Opcode);
+}
+
+/// Helper class that determines VL can use the same opcode.
+/// Alternate instruction is supported. In addition, it supports interchangeable
+/// instruction. An interchangeable instruction is an instruction that can be
+/// converted to another instruction with same semantics. For example, x << 1 is
+/// equal to x * 2. x * 1 is equal to x | 0.
+class BinOpSameOpcodeHelper {
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gbossu wrote:

Nit: that concept of equivalent/interchangeable opcode is rather orthogonal to vectorization, so it would be great to have it in separate source&header files, even if it's prefixed with `SLP` for now. It's just an idea, I guess there will be some push-back because currently `SLPVectorizer.cpp` contains all sorts of orthogonal things, but trimming it down into multiple files would really help vectorizer noobs like me 😄 

https://github.com/llvm/llvm-project/pull/133888