[llvm-commits] [llvm-gcc-4.2] r83229 - /llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp

[Duncan Sands]

Hi Bob,

> When copying an aggregate, always copy it element-by-element instead of using
> memcpy when the aggregate contains a single element, regardless of the size.
> For ARM, we set the size limit low (4 bytes) but the <arm_neon.h> header
> uses wrapper structs around NEON vector types, and then uses unions to
> convert those structs to and from the underlying vector types.  Those structs
> and unions should not be expanded to memcpy calls.

is this change for correctness, or an optimization?

>    // If the type is small, copy the elements instead of using a block copy.
> -  if (TREE_CODE(TYPE_SIZE(type)) == INTEGER_CST &&
> -      TREE_INT_CST_LOW(TYPE_SIZE_UNIT(type)) <
> -          TARGET_LLVM_MIN_BYTES_COPY_BY_MEMCPY) {
> -    const Type *LLVMTy = ConvertType(type);
> +  const Type *LLVMTy = ConvertType(type);
> +  unsigned NumElts = CountAggregateElements(LLVMTy);
> +  if (NumElts == 1 ||
> +      (TREE_CODE(TYPE_SIZE(type)) == INTEGER_CST &&
> +       TREE_INT_CST_LOW(TYPE_SIZE_UNIT(type)) <
> +       TARGET_LLVM_MIN_BYTES_COPY_BY_MEMCPY)) {

This will break structs with fields at variable offsets: when converting
to an LLVM struct type, all the fields at variable offsets are skipped.
So the LLVM struct may have only one field, but in fact there are many
more in the gcc type.  This is the reason for testing whether the size
of the type is a constant or not.  So please always check that the type
has constant size, regardless of the number of elements.

Ciao,

Duncan.