On Mon, 22 Jan 2024, Jeff Law wrote:
>
>
> On 1/15/24 06:34, Richard Biener wrote:
> > 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.
> >
> > Bootstrapped and tested on x86_64-unknown-linux-gnu (together
> > with 2/2). Jeff, can you maybe throw this on your tester?
> > Jakub, you did the PR64025 fix which was for a similar issue.
> No issues across the cross compilers with those two patches.
Thanks, pushed. I'm probably going to revert when bigger issues
appear (and hopefully we'd get some test coverage then).
Richard.
