On Mon, Nov 23, 2020 at 10:53 AM Marius Hillenbrand via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Hi,
>
> Digging into a test case failure with section anchors, I found
> dependence analysis return false negatives, leading to bad optimization
> by cse1. Two variables are synthetically constructed aliases. One is
> addressed relative to the section anchor and the other using a symbol
> ref, yet write_dependence_p() claims that they could not alias. I can
> reproduce that miscompile on aarch64, ppc64, and s390x. How is
> write_dependence_p() supposed to handle such cases with section anchors?
>
>
> Example: (simplified version of gcc.c-torture/execute/alias-2.c)
> int off; // causing nonzero offset of a to the section anchor
> int a;
> extern int b __attribute__ ((alias("a")));
> /* ... */
> int main()
> b=1; <-- uses symbol_ref to b
> a=2; <-- uses the section anchor + offset 4
> if (b!=2)
> __builtin_abort ();
> return 0;
> }
>
> so write_dependence gets to compare the canonicalized addresses
> (const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
> (const_int 4 [0x4])))
> and
> (symbol_ref:DI ("b") [flags 0x602] <var_decl b>)
> and
>
> I believe there are two ideas in the code, which could apply to this
> scenario: (a) identify anchor+4 being equal to the symbol_ref for a (in
> canonicalization?), (b) instead lookup (symbol_ref "b") -> a ->
> anchor+4, identify the shared section anchor and compare offsets
> (compare_base_symbol_refs appears to attempt that; yet find_base_term
> has dropped a's offset to the section anchor), or (c) that I am not
> aware of?
>
> How is that analysis supposed to drill into the addresses?
If the above is all we get to see then we have to give up when either
of the address is based on a section anchor and we know the other
one could possibly be refered through a section anchor.
In those cases we have to rely on MEM_EXPRs for disambiguation
which, for the anchored address, is unfortunately lost?
> (see also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92294 for the
> initial report on aarch64 and my full example)
>
> Marius
> --
> Marius Hillenbrand
> Linux on Z development
> IBM Deutschland Research & Development GmbH
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