On 08/20/18 12:41, Richard Biener wrote: > 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 alpha.md, >> 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 > *pbitsize, > 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? >
Yes, at first sight, however, I was involved at PR 58970, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58970 and I proposed a similar patch, which was objected by Jakub: see comment #25 of PR 58970: "Jakub Jelinek 2013-11-05 19:41:12 UTC (In reply to Bernd Edlinger from comment #24) > Created attachment 31169 [details] > https://gcc.gnu.org/bugzilla/attachment.cgi?id=31169 > Another (better) attempt at fixing this ICE. > > This avoids any negative bitpos from get_inner_reference. > These negative bitpos values are just _very_ dangerous all the > way down to expmed.c I disagree that it is better, you are forgetting get_inner_reference has other > 20 callers outside of expansion and there is no reason why negative bitpos would be a problem in those cases." So that is what Jakub said at that time, and with the suggested change in get_inner_reference, this part of the r204444 change would be effectively become superfluous: @@ -4721,6 +4721,15 @@ expand_assignment (tree to, tree from, bool nontem tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1, &unsignedp, &volatilep, true); + /* Make sure bitpos is not negative, it can wreak havoc later. */ + if (bitpos < 0) + { + gcc_assert (offset == NULL_TREE); + offset = size_int (bitpos >> (BITS_PER_UNIT == 8 + ? 3 : exact_log2 (BITS_PER_UNIT))); + bitpos &= BITS_PER_UNIT - 1; + } + if (TREE_CODE (to) == COMPONENT_REF && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1))) get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos, &offset); and should be reverted. I did not really like it then, but I'd respect Jakub's advice. Bernd. > Richard. >
