[mlir][AMDGPU] Allow non-contiguous destination memrefs for gather_to_lds

The requirement that the LDS operand is contiguous is overly restrictive
because it's perfectly valid to have a subview depend on subgroup IDs
that is still subgroup contiguous. We could continue trying to do this
verification based on the number of copied elements, but instead this
change just opts to clarify the semantics on the op definition.

Author: qedawkins

mlir/include/mlir/Dialect/AMDGPU/IR/AMDGPU.td

@@ -907,7 +907,8 @@ def AMDGPU_GatherToLDSOp :
       The elements gathered by the subgroup will be written contiguously in order of lane ID
       starting at `$dst[$dstIndices]`. Byte-sized (ex. i8) or short-sized (ex. i16)
       types will be zero-padded/extended to 32 bits before being written. 96-bit types
-      (ex. vector<3xf32>) will be zero-padded to 128 bits before being written.
+      (ex. vector<3xf32>) will be zero-padded to 128 bits before being written. Only the
+      offsets held by lane 0 are used.
     * `$transferType`: type of the data to be transferred by each thread. This is used to determine
       the size of the data to be transferred and the number of threads in the subgroup.
       The transfer type must be a scalar type or a vector type with a single element type.

mlir/lib/Dialect/AMDGPU/IR/AMDGPUDialect.cpp

@@ -518,9 +518,6 @@ LogicalResult GatherToLDSOp::verify() {
   MemRefType srcType = cast<MemRefType>(getSrc().getType());
   MemRefType dstType = cast<MemRefType>(getDst().getType());
 
-  if (!dstType.areTrailingDimsContiguous(dstType.getRank()))
-    return emitOpError("destination types must be contiguous");
-
   auto elemType = srcType.getElementType();
   // Check $src and $dst element types are the same.
   if (elemType != dstType.getElementType())

mlir/test/Dialect/AMDGPU/ops.mlir

@@ -539,13 +539,15 @@ func.func @transpose_load(%idx1 : index, %idx2 : index, %mem : memref<128x32xf16
 }
 
 // CHECK-LABEL: func @gather_to_lds
-func.func @gather_to_lds(%idx1 : index, %idx2 : index, %mem1 : memref<32xf16>, %mem2 : memref<32x32xf16>, %smem1 : memref<32xf16, #gpu.address_space<workgroup>>, %smem2 : memref<32x32xf16, #gpu.address_space<workgroup>>) {
+func.func @gather_to_lds(%idx1 : index, %idx2 : index, %mem1 : memref<32xf16>, %mem2 : memref<32x32xf16>, %smem1 : memref<32xf16, #gpu.address_space<workgroup>>, %smem2 : memref<32x32xf16, #gpu.address_space<workgroup>>, %smem3 : memref<?x?xf16, strided<[?, 1]>, #gpu.address_space<workgroup>>) {
   // CHECK: amdgpu.gather_to_lds %{{.*}}[%{{.*}}, %{{.*}}], %{{.*}}[%{{.*}}, %{{.*}}]
   // CHECK: amdgpu.gather_to_lds %{{.*}}[%{{.*}}, %{{.*}}], %{{.*}}[%{{.*}}]
   // CHECK: amdgpu.gather_to_lds %{{.*}}[%{{.*}}],          %{{.*}}[%{{.*}}, %{{.*}}]
+  // CHECK: amdgpu.gather_to_lds %{{.*}}[%{{.*}}],          %{{.*}}[%{{.*}}, %{{.*}}]
   amdgpu.gather_to_lds %mem2[%idx1, %idx2], %smem2[%idx1, %idx2] : vector<2xf16>, memref<32x32xf16>, memref<32x32xf16, #gpu.address_space<workgroup>>
   amdgpu.gather_to_lds %mem2[%idx1, %idx2], %smem1[%idx1]        : vector<2xf16>, memref<32x32xf16>, memref<32xf16,    #gpu.address_space<workgroup>>
   amdgpu.gather_to_lds %mem1[%idx1],        %smem2[%idx1, %idx2] : vector<2xf16>, memref<32xf16>,    memref<32x32xf16, #gpu.address_space<workgroup>>
+  amdgpu.gather_to_lds %mem1[%idx1],        %smem3[%idx1, %idx2] : vector<2xf16>, memref<32xf16>,   memref<?x?xf16, strided<[?, 1]>, #gpu.address_space<workgroup>>
   func.return
 }