I have been working on AVR port and have come across many instances
where poor code is produced due to the absence of effective forward
propagation of operands before register allocation.
The AVR target in particular benefits from register lowering pass as
many physical registers and instructions are only 8 bits. However, most
of the opportunities this creates for register and instruction
elimination are not realized as the only following pass that can help
is register allocation which has minimal propagation capabilities - and
no instruction simplifications.
Similarly, instruction splitting created at the combine stage and
before reload do not reliase the expected benefits from spliiting into 8
bit operations. In fact if any new pseudo are created by a split this
often turn into new hard register.
Targets other than the AVR which have small sized registers (i86 and
68HC11 for example) are likely to have the same issues.
There have been suggestions in tha pass to split all instructions by RTL
expanders. Indeed I and others have explored this. However, this has
proved fruitless as this also requires a change to a non-CC0 target. At
this time this cannot be realized due to problems reloading addresses
and reloading arithmetic carry operations.
Hybrid approaches that split some instructions using RTL expanders have
also been tried. But the resultant complexity (some split -some are
not) then defeats early RTL optimizations -so we loose more than we gain.
With an additional forward propagation pass prior to register
allocation the full benefits of Subreg lowering and splitting are obtained.
I realise than nobody is keen on new passes. Yet I can see no other way
in which this problem can be addressed.
From a target viewpoint, moving fwprop2 latter would be fine. Adding a
target dependent pass (before reload) would be also be fine. Indeed any
way we could get better forward propagation/simplification after
combine/lowering and before register allocation would be great.
Can I humbly ask that the maintainers of gcc seriously consider this
request and provide some means by which to solve the issue.
regards
Andy
