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Some questions about pass web
- From: Carrot Wei <carrot at google dot com>
- To: hubicka at ucw dot cz, GCC Development <gcc at gcc dot gnu dot org>
- Date: Tue, 2 Sep 2014 16:35:12 -0700
- Subject: Some questions about pass web
- Authentication-results: sourceware.org; auth=none
I have following questions about web (pseudo register renaming) pass:
1. It is well known that register renaming is a big help to register
allocation, but in gcc's backend, the web pass is far before RA, there
are about 20 passes between them. Does it mean register renaming can
also heavily benefit other optimizations? And the passes between them
usually don't generate more register renaming chances?
2. It looks current web pass can't handle AUTOINC expressions, a reg
operand is used as both use and def reference in an AUTOINC
expression, so this def side should not be renamed. Pass web doesn't
explicitly check this case, may rename the reg operand of AUTOINC
expression. Is this expected because it is before auto_inc_dec pass?
3. Are AUTOINC expressions only generated by auto_inc_dec pass? All
passes before auto_inc_dec including expand should not generate
AUTOINC expressions, otherwise it will break web.
Could anybody help to answer these questions?
thanks a lot