[clang] [CIR] Make cir.alloca alignment mandatory with default 0 (PR #172663)

[Andy Kaylor via cfe-commits]

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@@ -530,7 +530,7 @@ def CIR_AllocaOp : CIR_Op<"alloca", [
     StrAttr:$name,
     UnitAttr:$init,
     UnitAttr:$constant,
-    ConfinedAttr<OptionalAttr<I64Attr>, [IntMinValue<0>]>:$alignment,
+    ConfinedAttr<DefaultValuedAttr<I64Attr, "0">, [IntMinValue<0>]>:$alignment,
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andykaylor wrote:

In that case, we're providing the alignment in the call to `emitAlloca` (via the `CharUnits()` argument). We're doing it wrong, but that's where we're doing it, so it has nothing to do with the AllocaOp needing to provide a default value.

The equivalent code in classic codegen calls the `llvm::AllocaInst` constructor directly without an alignment argument, and that in turn calls `computeAllocaDefaultAlign`. We should be doing something equivalent to that in `createTempAlloca` (and any other place that doesn't reasonably compute the alignment). I think that's beyond the scope of this PR.

https://github.com/llvm/llvm-project/pull/172663