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Re: [PATCH] Avoid negative bitpos in expand_expr_real_1 (PR 86984)

On Sun, 19 Aug 2018, Bernd Edlinger wrote:

> Hi!
> This fixes a wrong code issue in expand_expr_real_1 which happens because
> a negative bitpos is actually able to reach extract_bit_field which
> does all computations with poly_uint64, thus the offset 0x1ffffffffffffff0.
> To avoid that I propose to use Jakub's r204444 patch from the expand_assignment
> also in the expand_expr_real_1.
> This is a rather unlikely thing to happen, as there are lots of checks that are
> of course all target dependent between the get_inner_reference and the
> actual extract_bit_field call, and all other code paths may or may not have a problem
> with negative bit offsets.  Most don't have a problem, but the bitpos needs to be
> folded into offset before it is used, therefore it is necessary to handle the negative
> bitpos very far away from the extract_bit_field call.  Doing that later is IMHO not
> possible.
> The fix in CONSTANT_ADDRESS_P is actually unrelated, and I only spotted it because
> this macro is used in alpha_legitimize_address which is of course what one looks
> at first if something like that happens.
> I think even with this bogus offset it should not have caused a linker error, so there
> is probably a second problem in the *movdi code pattern of the, because it
> should be split into instructions that don't cause a link error.
> Once the target is fixed to split the impossible assembler instruction, the test case
> will probably no longer be able to detect the pattern in the assembly.
> Therefore the test case looks both at the assembler output and the expand rtl dump
> to spot the bogus offset.  I only check the first 12 digits of the bogus constant,
> because it is actually dependent on the target configuration:
> I built first a cross-compiler without binutils, and it did used a slightly different
> offset of 0x2000000000000000, (configured with: --target=alpha-linux-gnu --enable-languages=c
> --disable-libgcc --disable-libssp --disable-libquadmath --disable-libgomp --disable-libatomic)
> when the binutils are installed at where --prefix points, the offset is 0x1ffffffffffffff0.
> Regarding the alpha target, I could not do more than build a cross compiler and run
> make check-gcc-c RUNTESTFLAGS="alpha.exp=pr86984.c".
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?

Hmm, I don't remember why we are inconsistent in get_inner_reference
with respect to negative bitpos.  I think we should be consistent
here and may not be only by accident?  That is, does

diff --git a/gcc/expr.c b/gcc/expr.c
index c071be67783..9dc78587136 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -7272,7 +7272,7 @@ get_inner_reference (tree exp, poly_int64_pod 
                                      TYPE_PRECISION (sizetype));
       tem <<= LOG2_BITS_PER_UNIT;
       tem += bit_offset;
-      if (tem.to_shwi (pbitpos))
+      if (tem.to_shwi (pbitpos) && !maybe_lt (*pbitpos, 0))
        *poffset = offset = NULL_TREE;
fix the issue?


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