ExprParserWriter

🚧 NOTE: This is a work in progress, variables are not yet bound and there is at least one BUG in it. 🚧

ExprParserWriter is a parser / re-writer for the C-like expressions that are used in the expr/expr~ objects of the PureData programming language.

Its intended purpose is for the continued development of the Wasted Audio fork of the Heavy/HVCC PureData compiler which will turn a valid Pd patch into executable C-code. Below is an example of a simple expression and the resulting nested C-code and sequential SIMD ready C-code that is generated by this tool.

// input expression
sin($f1 + 2) / sqrt($f2)

// generated nested C-code
hv_div_f(hv_sin_f(hv_add_f($f1, 2)), hv_sqrt_f($f2));

// generated sequential SIMD ready C-code
__hv_add_f($f1, 2, BO0);
__hv_sin_f(BO0, BO1);
__hv_sqrt_f($f2, BO0);
__hv_div_f(BO1, BO0, BO2);

Using it (this is still in the works...)

from c_expr_writer import CExprWriter

expression = "sin($f1 + 2) / sqrt($f2)"
c_code_lines = CExprWriter(expression).to_c_simd(input_buffers, output_buffer)
# will output the generated code above

Why...?

Pd provides two versions of expr: [expr] and [expr~] where the ~ indicates that the object runs at "signal rate". Objects without the ~ are run at control rate which is a couple orders of magnitude slower.

Because of this difference in needed execution speed, the audio-rate objects rely on heavily optimized code to ensure that they can run at full speed without buffer underflows. This is a case where it can never be fast enough because further improvements will always allow a user to do more with a given piece of hardware. It should be noted that smaller ARM μ-controllers are a common target for HVCC generated code, so being precious with resources is a requirement.

The existing HVCC infrastructure provides wrappers for most of the native C operators and functions from math.h of the form hv_FUNC_f() or __hv_FUNC_f(), for example: hv_add_f(a, b) or __hv_add_f(a, b, out). They are designed this way to facilitate compiling for different SIMD instruction sets, namely: AVX and SSE for the x86 architecture and NEON for ARM. If none of these SIMD optimizations are applied the code falls back to the native C function calls. The functions with the leading __ are the SIMD ready functions and the functions without can be thought of as straight wrappers around the C library functions.