Granular `@fastmath` flags

[mikmoore]

@fastmath is currently an all-or-nothing construct: one opts into every available flag (which is subject to change with new LLVM versions) by using this macro. There is no syntax to invoke a specific subset of the available flags.

A syntax for opt-in flags like @fastmath (;reassoc=true,contract=true,nsz=true) EXPR would be welcome for a more controlled @fastmath experience.

[giordano]

For the record, tangentially related to #49387 and #49405 (comment)

[longemen3000]

maybe something like the @assume_effects macro?

@fastmath :contract :reassoc Expr #for applying those flags to the expr
@fastmath Expr #default behaviour, all flags on

[ajwheeler]

I started looking into this yesterday, and I see two possible ways forward.

One option: the fast_func intrinsics could be modified to optionally take an encoding of the fast-math flags as an additional argument. Then, @fastmath just has to encode the desired flags and recursively rewrite the AST, as it does now.

Alternatively, this seems like the kind of thing that :meta was intended for. For backwards compatibility, all the fast math functions (FastMath.fast_whatever) could be reimplemented in terms of @fastmath instead of the other way around. Doing things this way would be more involved, but it might be "better"?

I don't feel fully equipped to evaluate the tradeoffs, as I'm a novice when it comes to the compiler internals.