[PATCH, GCC] PR target/86487: fix the way 'uses_hard_regs_p' handles paradoxical subregs

[Andre Vieira (lists)]

Hi,

This patch fixes the way 'uses_hard_regs_p' handles paradoxical subregs. 
  The function is supposed to detect whether a register access of 'x' 
overlaps with 'set'.  For SUBREGs it should check whether any of the 
full multi-register overlaps with 'set'.  The former behavior used to 
grab the widest mode of the inner/outer registers of a SUBREG and the 
inner register, and check all registers from the inner-register onwards 
for the given width.  For normal SUBREGS this gives you the full 
register, for paradoxical SUBREGS however it may give you the wrong set 
of registers if the index is not the first of the multi-register set.

The original error reported in PR target/86487 can no longer be 
reproduced with the given test, this was due to an unrelated code-gen 
change, regardless I believe this should still be fixed as it is simply 
wrong behavior by uses_hard_regs_p which may be triggered by a different 
test-case or by future changes to the compiler.  Also it is useful to 
point out that this isn't actually a 'target' issue as this code could 
potentially hit any other target using paradoxical SUBREGS.  Should I 
change the Bugzilla ticket to reflect this is actually a target agnostic 
issue in RTL?

There is a gotcha here, I don't know what would happen if you hit the 
cases of get_hard_regno where it would return -1, quoting the comment 
above that function "If X is not a register or a subreg of a register, 
return -1." though I think if we are hitting this then things must have 
gone wrong before?

Bootstrapped on aarch64, arm and x86, no regressions.

Is this OK for trunk?


gcc/ChangeLog:
2019-01-07 Andre Vieira  <andre.simoesdiasvieira@arm.com>


         PR target/86487
         * lra-constraints.c(uses_hard_regs_p): Fix handling of 
paradoxical SUBREGS.
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