arsenm added a comment.
In D89980#2357208 <https://reviews.llvm.org/D89980#2357208>, @hliao wrote:
> Besides the unpromotable `alloca` issue due to indirect accesses, such
> coercion to GLOBAL pointer directly is not safe as, in HIP/CUDA, both
> CONSTANT and GLOBAL pointers would be passed as the kernel arguments. Without
> introducing a new address space combing GLOBAL/CONSTANT, such coercion would
> be unsafe.
I'm not sure what you are saying here. Constant address space doesn't really
exist and we could eliminate it. It is always valid to use global in place of
constant.
================
Comment at: clang/test/CodeGenCUDA/amdgpu-kernel-arg-pointer-type.cu:19
+// COMMON-LABEL: define amdgpu_kernel void @_Z7kernel1Pi(i32*{{.*}} %x)
+// OPT: [[VAL:%.*]] = load i32, i32* %x, align 4
// OPT: [[INC:%.*]] = add nsw i32 [[VAL]], 1
----------------
hliao wrote:
> arsenm wrote:
> > hliao wrote:
> > > arsenm wrote:
> > > > hliao wrote:
> > > > > arsenm wrote:
> > > > > > This is still a regression. Fixing up AA does not solve the problem
> > > > > > this promotions this is intended to solve. Generic accesses are
> > > > > > worse independently of the aliasing properties
> > > > > Do you mean FLAT load/store has worse addressing mode than GLOBAL
> > > > > ones?
> > > > Yes. The flat offsets have a smaller range, and do not have the saddr
> > > > mode. Flat accesses also won't avoid the extra lgmkcnt wait
> > > I plan to add support to select GLOBAL ones once we could confirm that
> > > pointer could only point to GLOBAL/CONSTANT address spaces. Do you think
> > > that's a reasonable solution?
> > I would much rather have the IR express the address space rather than
> > fixing it up later. IR passes are aware of the addressing mode differences.
> > Relying on AA for basic selection would also be worse for compile time
> LLVM IR is agnostic to the underlying addressing mode. In code selection, we
> won't use AA but just needs to check IR.
It's not really. The codegen IR passes check the addressing mode based on the
indexed address space. Using a flat pointer is strictly worse than an
addrspace(1) pointer
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