hliao added a comment.
In D89980#2368506 <https://reviews.llvm.org/D89980#2368506>, @arsenm wrote:
> I think this is a dead end approach. I don't see the connection to the
> original problem you are trying to solve. Can you send me an IR testcase that
> this is supposed to help?
That's probably commonly known. If we pass an aggregate parameter directly by
value and dynamically index it late, that `alloca` cannot be promoted as that
aggregate value in LLVM IR cannot be dynamically indexed. For example,
struct S {
int a[100];
int n;
};
int foo(S s) {
return s.a[s.n];
}
If the underlying ABI chooses to pass `s` directly by value, we have the
following pseudo IR.
%s = alloca S
; store `s` value into %s as the parameter is treated as a local variable by
filling its initial value from LLVM IR parameter.
...
; regular parameter access through %s with dynamic indices
that `store` from the parameter from LLVM IR is an aggregate value store.
Later, when %s is to be promoted, as it's once dynamically indexed, we cannot
promote it as dynamic index on aggregate values is not representable in LLVM IR.
In contrast, if a parameter is passed by value indirectly, that `store` is
replaced with a `memcpy`. It's straightforward to promote '%s' as they are all
memory operands of the same layout.
If you need detailed IR, I may post here for your reference.
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D89980/new/
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