On Wed, Sep 3, 2014 at 7:35 AM, Carrot Wei <car...@google.com> wrote: > Hi > > 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? I think one purpose is to break long dependency chain into short ones. For example, with below code
use(i) i = i + 1; ... use(i) i = i + 1; ... use(i) i = i + 1; ... Pass fweb could change it into below form use(i) i0 = i + 1 ... use(i0) i1 = i0 + 1 ... use(i1) i = i0 + 2 ... Apparently, latter form has shorter chains, which makes df stuff more efficient. > > 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? Last time I tried, there are several passes after loop_done and before auto-inc-dec can't handle auto-increment addressing mode, including fweb. > > 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. Yes. Yet other passes may generate auto-inc friendly instruction patterns thus auto-inc-dec can capture more opportunities. IVOPT is a typical example. Thanks, bin > > Could anybody help to answer these questions? > > thanks a lot > Guozhi Wei
