Re: PR 27073: invalid gcse manipulation of REG_EQUIV notes

[Roger Sayle <roger at eyesopen dot com>]

Hi Richard,

On Sun, 9 Apr 2006, Richard Sandiford wrote:
> I didn't mention any potential performance problems. ;)

My apologies, I'm one of those paranoid folks who immediately equate
changes in register allocation with potential performance problems;
you did write:

>> this is bound to be a regression for _some_ testcase, due to
>> differences in register allocation.



> I don't understand.  Why is this any different from what my
> patch does?

The difference cconcerns this test from later in try_replace_reg:

      if (!success && note == 0 && set != 0

Previously for instructions containing REG_EQUIV notes, note would
be non-zero, however it now indicates whether we had a REG_EQUAL
note.  This will now lead to situations where our instruction will
have both REG_EQUAL and REG_EQUIV notes, as GCSE may now blindly
add a REG_EQUAL note if the substitution isn't a valid instruction.
This may or may not confuse later passes, for example the function
find_reg_equal_equiv_note assumes there's only or other note, and
reload changes notes from one form to another, which will result
in multiple notes.


The potential performance impact was that previously we were making
unsafe unsubstitutions in REG_EQUIV notes, but in some circumstances
they may have been beneficial.  For example, allowing us to eliminate
all references to a hard frame pointer.  Whilst I agree that your
patch is a correctness fix (nothing guarantees the replacement is
valid throughout the function), there may exists codes that have
previously benefited from their luck.  If they never spill the
stack parameter, they'll never encounter problems.  Either way,
the compiler may now be generating different code on different
targets.


Not that I anticipate problems with your patch, or I wouldn't
have approved it.  It's that there may be subtleties in this
change, and alternate fixes that look identical at first glance,
may have slightly different "therapeutic profiles" of downstream
side-effects.

Roger
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