Commits on May 10, 2024

1. [mlir][tosa] Work around GCC bug in tosa-to-tensor …

   GCC 12 and 13 generate incorrect code for a pattern in the
   tosa-to-tensor pass responsible for lowering tosa.reshape.
   This results in the tosa.reshape lowering producing IR which fails to
   verify. I've narrowed down the set of cmake flags needed to reproduce
   the issue to this:
   
       cmake -G Ninja ../llvm \
         -DLLVM_ENABLE_PROJECTS="mlir" \
         -DLLVM_TARGETS_TO_BUILD=host \
         -DLLVM_ENABLE_PROJECTS=mlir \
         -DCMAKE_BUILD_TYPE="Release" \
         -DCMAKE_CXX_FLAGS_RELEASE="-O2" \
         -DCMAKE_CXX_FLAGS="-O2" \
         -DCMAKE_CXX_COMPILER=g++ \
         -DCMAKE_C_COMPILER=gcc
   
   This is the failing test case:
   
       func.func @fails_in_gcc_12(%arg0: tensor<?xf32>) -> tensor<1x1x1x?xf32> {
         %0 = tosa.reshape %arg0 {new_shape = array<i64: 1, 1, 1, -1>} : (tensor<?xf32>) -> tensor<1x1x1x?xf32>
         return %0 : tensor<1x1x1x?xf32>
       }
   
   This should correctly lower to a single tensor.expand_shape operation
   like so:
   
       func.func @foo(%arg0: tensor<?xf32>) -> tensor<1x1x1x?xf32> {
         %c0 = arith.constant 0 : index
         %dim = tensor.dim %arg0, %c0 : tensor<?xf32>
         %c1 = arith.constant 1 : index
         %expanded = tensor.expand_shape %arg0 [[0, 1, 2, 3]] output_shape [1, 1, 1, %dim] : tensor<?xf32> into tensor<1x1x1x?xf32>
         return %expanded : tensor<1x1x1x?xf32>
       }
   
   Under GCC 12/13 with the above cmake configuration, the tensor.expand_shape
   looks like this
   
       %2 = "tensor.expand_shape"(%arg0) <{reassociation = [[0, 1, 2, 3]], static_output_shape = array<i64>}> : (tensor<?xf32>) -> tensor<?x1x1x?xf32>
   
   This expand_shape fails to verify with this error message:
   
       error: 'tensor.expand_shape' op expected number of static shape dims to be equal to the output rank (4) but found 0 inputs instead
   
   The problematic code is calculating the intermediate shape of the
   generated tensor.expand_shape operation in the
   expand_shape/collapse_shape sequence that implements tosa.reshape.
   
       // Compute result shape
       bool resultIsStatic = true;
       auto resultShape = llvm::map_to_vector(newShape, [&](int64_t size) {
         // Omitted
   
         // If we do not know the total size of the tensor, keep this dimension
         // dynamic in the result shape.
         if (!inputIsStatic) {
           resultIsStatic = false;
           return ShapedType::kDynamic;
         }
       });
   
       if (resultIsStatic) {
         // do something
         return;
       }
   
       // do something else
       return;
   
   The failure point seems to be the update of the resultIsStatic variable
   in the lambda body. The assignment of false is not propagated to the use
   in the if-statement, resulting in the branch being taken when it should
   not.
   
   I've found several modification to the code that gets around the bug.
   The version I settled on is one which makes the logic a little more
   obvious.

   

   sabauma authored and Spenser Bauman committed May 10, 2024
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