rtl-optimization/113255 - base_alias_check vs. pointer difference

When the x86 backend generates code for cpymem with the rep_8byte
strathegy for the 8 byte aligned main rep movq it needs to compute
an adjusted pointer to the source after doing a prologue aligning
the destination.  It computes that via

  src_ptr + (dest_ptr - orig_dest_ptr)

which is perfectly fine.  On RTL this is then

    8: r134:DI=const(`g'+0x44)
    9: {r133:DI=frame:DI-0x4c;clobber flags:CC;}
      REG_UNUSED flags:CC
   56: r129:DI=const(`g'+0x4c)
   57: {r129:DI=r129:DI&0xfffffffffffffff8;clobber flags:CC;}
      REG_UNUSED flags:CC
      REG_EQUAL const(`g'+0x4c)&0xfffffffffffffff8
   58: {r118:DI=r134:DI-r129:DI;clobber flags:CC;}
      REG_DEAD r134:DI
      REG_UNUSED flags:CC
      REG_EQUAL const(`g'+0x44)-r129:DI
   59: {r119:DI=r133:DI-r118:DI;clobber flags:CC;}
      REG_DEAD r133:DI
      REG_UNUSED flags:CC

but as written find_base_term happily picks the first candidate
it finds for the MINUS which means it picks const(`g') rather
than the correct frame:DI.  This way find_base_term (but also
the unfixed find_base_value used by init_alias_analysis to
initialize REG_BASE_VALUE) performs pointer analysis isn't
sound.  The following restricts the handling of multi-operand
operations to the case we know only one can be a pointer.

This for example causes gcc.dg/tree-ssa/pr94969.c to miss some
RTL PRE (I've opened PR113395 for this).  A more drastic patch,
removing base_alias_check results in only gcc.dg/guality/pr41447-1.c
regressing (so testsuite coverage is bad).  I've looked at
gcc.dg/tree-ssa tests and mostly scheduling changes are present,
the cc1plus .text size is only 230 bytes worse.  With the this
less drastic patch below most scheduling changes are gone.

x86_64 might not the very best target to test for impact, but
test coverage on other targets is unlikely to be very much better.

	PR rtl-optimization/113255
	* alias.cc (find_base_term): Remove PLUS/MINUS handling
	when both operands are not CONST_INT_P.

	* gcc.dg/torture/pr113255.c: New testcase.

0 files changed, 0 insertions, 0 deletions