compiler/cmm/Bitmap.hs¶
Note [Strictness when building Bitmaps]¶
One of the places where @Bitmap@ is used is in in building Static Reference Tables (SRTs) (in @CmmBuildInfoTables.procpointSRT@). In #7450 it was noticed that some test cases (particularly those whose C– have large numbers of CAFs) produced large quantities of allocations from this function.
The source traced back to ‘intsToBitmap’, which was lazily subtracting the word size from the elements of the tail of the @slots@ list and recursively invoking itself with the result. This resulted in large numbers of subtraction thunks being built up. Here we take care to avoid passing new thunks to the recursive call. Instead we pass the unmodified tail along with an explicit position accumulator, which get subtracted in the fold when we compute the Word.
