hppa: Fix handling of unscaled index addresses on HP-UX

The PA-RISC architecture uses the top two bits of memory pointers
to select space registers.  The space register ID is ored with the
pointer offset to compute the global virtual address for an access.

The new late combine passes broke gcc on HP-UX.  One of these passes
runs after reload.  The existing code assumed no unscaled index
instructions would be created after reload as the REG_POINTER flag
is not reliable after reload.  The new pass sometimes interchanged
the base and index registers, causing these instructions to fault
when the wrong space register was selected.

I investigated various alternatives to try to retain generation
of unscaled index instructions on HP-UX.  It's not possible to
simply treat unscaled index addresses as not legitimate after
reload as sometimes instructions need to be rerecognized after
reload.  So, we needed to allow unscaled index addresses after
reload and to disable the late combine passes.

I had noticed that reversing the current order of base and index
register canonicalization resulted in more accesses using unscaled
index addresses.  However, this exposed issues with the REG_POINTER
flag.

The flag is not propagated when a constant is added to a pointer.
Tree opimization sometimes adds two pointers.  I found that I had
to treat the result as a pointer but the addition generally corrupts
the space register bits.  These get fixed when a negative pointer
is added.  Finally, the REG_POINTER flag isn't set when a pointer
is passed in a function call.  I couldn't get this approach to work.

Thus, I came to the conclusion that the best approach was to
disable use of unscaled index addresses on HP-UX.  I don't think
this impacts performance significantly.  Code size might get
slightly larger but we get some or more back from having the late
combine passes.

2024-08-29  John David Anglin  <danglin@gcc.gnu.org>

gcc/ChangeLog:

	* config/pa/pa.cc (load_reg): Don't generate load with
	unscaled index address when !TARGET_NO_SPACE_REGS.
	(pa_legitimate_address_p): Only allow unscaled index
	addresses when TARGET_NO_SPACE_REGS.

0 files changed, 0 insertions, 0 deletions