[llvm] [AMDGPU] Extend promotion of alloca to vectors (PR #127973)

[Carl Ritson via llvm-commits]

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@@ -66,6 +66,18 @@ static cl::opt<unsigned> PromoteAllocaToVectorLimit(
     cl::desc("Maximum byte size to consider promote alloca to vector"),
     cl::init(0));
 
+static cl::opt<unsigned> PromoteAllocaToVectorMaxElements(
+    "amdgpu-promote-alloca-to-vector-max-elements",
+    cl::desc("Maximum vector size (in elements) to use when promoting alloca"),
+    cl::init(16));
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perlfu wrote:

> Elements seems like a strange way to express this.

Element count is how the limit is currently defined in the code.  I agree in terms of 32-bit words (registers) would make more sense.  I'll change to this model, but it does mean this patch will not preserve the existing limit so some edge case promotion will change.

> Do we expect end users to use these options?

Graphics front end will use these for shader tuning, which is why they are accessible via function attributes as well.

https://github.com/llvm/llvm-project/pull/127973