diff --git a/llvm/docs/AMDGPUUsage.rst b/llvm/docs/AMDGPUUsage.rst index ba0e53bceade8..b8b372d4113c1 100644 --- a/llvm/docs/AMDGPUUsage.rst +++ b/llvm/docs/AMDGPUUsage.rst @@ -1016,6 +1016,15 @@ supported for the ``amdgcn`` target. `ptr addrspace(7)` directly, which produces a buffer fat pointer with an initial offset of 0 and prevents the address space cast from being rewritten away. + The ``align`` attribute on operations from buffer fat pointers is deemed to apply + to all componenents of the pointer - that is, an ``align 4`` load is expected to + both have the offset be a multiple of 4 and to have a base pointer with an + alignment of 4. + + This componentwise definition of alignment is needed to allow for promotion of + aligned loads to ``s_buffer_load``, which requires that both the base pointer and + offset be appropriately aligned. + **Buffer Resource** The buffer resource pointer, in address space 8, is the newer form for representing buffer descriptors in AMDGPU IR, replacing their @@ -1039,6 +1048,25 @@ supported for the ``amdgcn`` target. (bits `127:96`). The specific interpretation of these fields varies by the target architecture and is detailed in the ISA descriptions. + When buffer resources are passed to buffer intrinsics such as + ``llvm.amdgcn.raw.ptr.buffer.load`` or + ``llvm.amdgcn.struct.ptr.buffer.store``, the ``align`` attribute on the + pointer is assumed to apply to both the offset and the base pointer value. + That is, ``align 8`` means that both the base address within the ``ptr + addrspace(8)`` and the ``offset`` argument have their three lowest bits set + to 0. If the stride of the resource is nonzero, the stride must be a multiple + of the given alignment. + + In other words, the ``align`` attribute specifies the alignment of the effective + address being loaded from/stored to *and* acts as a guarantee that this is + not achieved from adding lower-alignment parts (as hardware may not always + allow for such an addition). For example, if a buffer resource has the base + address ``0xfffe`` and is accessed with a ``raw.ptr.buffer.load`` with an offset + of ``2``, the load must **not** be marked ``align 4`` (even though the + effective adddress ``0x10000`` is so aligned) as this would permit the compiler + to make incorrect transformations (such as promotion to ``s_buffer_load``, + which requires such componentwise alignment). + **Buffer Strided Pointer** The buffer index pointer is an experimental address space. It represents a 128-bit buffer descriptor and a 32-bit offset, like the **Buffer Fat @@ -1057,6 +1085,12 @@ supported for the ``amdgcn`` target. index and offset values are both 0. This prevents the address space cast from being rewritten away. + As with buffer fat pointers, alignment of a buffer strided pointer applies to + both the base pointer address and the offset. In addition, the alignment also + constrains the stride of the pointer. That is, if you do an ``align 4`` load from + a buffer strided pointer, this means that the base pointer is ``align(4)``, that + the offset is a multiple of 4 bytes, and that the stride is a multiple of 4. + **Streamout Registers** Dedicated registers used by the GS NGG Streamout Instructions. The register file is modelled as a memory in a distinct address space because it is indexed