I have a best-practice kind of question:
The x86_64 System V ABI passes floating point values in the xmm0, xmm1, ...
128-bit SSE vector registers. I’m wondering what the correct DWARF v4 (lacking
the DWARF 5 type conversions) expression for a 32-bit float in xmm0 is.
Given the following program:
float return_float() __attribute__((noinline)) { return 3.14f; }
int main(int argc, char** argv) {
float f = return_float();
return (int)f;
}
Clang (-O1) currently produces the following DWARF for the variable f:
DW_TAG_variable
DW_AT_location( reg17 ) // xmm0 = 17
DW_AT_name( "f" )
DW_AT_decl_file( "test.c" )
DW_AT_decl_line( 8 )
DW_AT_type( {0x0000004a} ( float ) )
Which causes LLDB to complain that the size of register xmm0 (128 Bits) is
different from the size of f (32 Bits). I wonder who is wrong? Is LLDB being
too strict, or should clang actually emit something like DW_OP_reg 17 (xmm0)
DW_OP_piece 4 to make it abundantly clear that only the lower 32 bits of the
vector register are interesting?
-- adrian
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