Ppr: compute length of string literals at compile time (#19266)
Commit message:
SDoc string literals created for example with `text "xyz"` are converted
into `PtrString` (`Addr#` + size in bytes) with a rewrite rule to avoid
allocating a String.
Before this patch, the size in bytes was still computed at runtime. For
every literal, we obtained the following pseudo STG:
x :: Addr#
x = "xzy"#
s :: PtrString
s = \u [] case ffi:strlen [x realWorld#] of
(# _, sz #) -> PtrString [x sz]
But since GHC 9.0, we can use `cstringLength#` instead to get:
x :: Addr#
x = "xzy"#
s :: PtrString
s = PtrString! [x 3#]
Literals become statically known constructor applications. Allocations
seem to decrease a little in perf tests (between -0.1% and -0.7% on CI).
Edited by Sylvain Henry