> Am 30.07.2026 um 23:32 schrieb Jakub Jelinek <[email protected]>:
>
> Hi!
>
> The following testcase is miscompiled on aarch64 (but not on x86_64).
> The difference is that x86_64/i686 define optabs that make it use IFN_UADDC
> and IFN_USUBC, those are then used both in the loop and to perform the
> most significant limb, so
> # _6 = PHI <0(2), _7(3)>
> # _9 = PHI <0(2), _10(3)>
> _8 = VIEW_CONVERT_EXPR<unsigned long[5]>(a)[_6];
> _11 = .USUBC (0, _8, _9);
> _12 = IMAGPART_EXPR <_11>;
> _13 = REALPART_EXPR <_11>;
> VIEW_CONVERT_EXPR<unsigned long[7]>(<retval>)[_6] = _13;
> _14 = _6 + 1;
> _15 = VIEW_CONVERT_EXPR<unsigned long[5]>(a)[_14];
> _16 = .USUBC (0, _15, _12);
> _10 = IMAGPART_EXPR <_16>;
> _17 = REALPART_EXPR <_16>;
> VIEW_CONVERT_EXPR<unsigned long[7]>(<retval>)[_14] = _17;
> _7 = _6 + 2;
> if (_7 != 4)
> in the loop and
> _18 = MEM <unsigned long> [(_BitInt(257) *)&a + 32B];
> _19 = (<unnamed-signed:1>) _18;
> _20 = (<unnamed-unsigned:1>) _19;
> _21 = (unsigned long) _20;
> _22 = .USUBC (0, _21, _10);
> _23 = IMAGPART_EXPR <_22>;
> _24 = REALPART_EXPR <_22>;
> _25 = (<unnamed-signed:1>) _24;
> _26 = (unsigned long) _25;
> MEM <unsigned long> [(unsigned _BitInt(400) *)&<retval> + 32B] = _26;
> ...
> after the loop. Now, on targets which don't support the optab, we instead
> use
> # _6 = PHI <0(2), _7(3)>
> # _9 = PHI <0(2), _10(3)>
> _8 = VIEW_CONVERT_EXPR<unsigned long[6]>(a)[_6];
> _11 = .SUB_OVERFLOW (0, _8);
> _13 = REALPART_EXPR <_11>;
> _14 = IMAGPART_EXPR <_11>;
> _15 = .SUB_OVERFLOW (_13, _9);
> _16 = IMAGPART_EXPR <_15>;
> _12 = _14 + _16;
> _17 = REALPART_EXPR <_15>;
> VIEW_CONVERT_EXPR<unsigned long[8]>(<retval>)[_6] = _17;
> _18 = _6 + 1;
> _19 = VIEW_CONVERT_EXPR<unsigned long[6]>(a)[_18];
> _20 = .SUB_OVERFLOW (0, _19);
> _21 = REALPART_EXPR <_20>;
> _22 = IMAGPART_EXPR <_20>;
> _23 = .SUB_OVERFLOW (_21, _12);
> _24 = IMAGPART_EXPR <_23>;
> _10 = _22 + _24;
> _25 = REALPART_EXPR <_23>;
> VIEW_CONVERT_EXPR<unsigned long[8]>(<retval>)[_18] = _25;
> _7 = _6 + 2;
> if (_7 != 4)
> in the loop (i.e. instead of one .USUBC 2 .SUB_OVERFLOW) and then
> after the loop for the most significant limb
> _26 = MEM <unsigned long> [(_BitInt(257) *)&a + 32B];
> _27 = (<unnamed-signed:1>) _26;
> _28 = (<unnamed-signed:1>) _10;
> _29 = 0 - _27;
> _30 = _29 - _28;
> _31 = (unsigned long) _30;
> MEM <unsigned long> [(unsigned _BitInt(400) *)&<retval> + 32B] = _31;
> Now, the last thing is what is wrong. We need to do two subtractions
> (or after folding one negation and one subtraction), and while in the
> original operation signed overflow is indeed undefined, it just means
> that the two operations together don't overflow, but one of them can.
> In this testcase (in foo function) on aarch64, _26 is 1 (the most
> significant bit of 257-bit negative value) and _10 is also 1 (borrow
> from within the loop). When we perform this computation in signed 1-bit
> precision, we have 0 - -1 (overflow) and -1 - -1 (another overflow).
> The RTL emitted for this then results in miscompilation, but we really
> shouldn't introduce UB into the IL for something that didn't have UB
> originally.
>
> So, the following patch forces use of unsigned type for these and casts
> to the signed one only at the end.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, bootstrapped
> on aarch64-linux too, ok for trunk/16.2 if it passes there as well?
Ok
Richard
> 2026-07-30 Jakub Jelinek <[email protected]>
>
> PR tree-optimization/126503
> * gimple-lower-bitint.cc (bitint_large_huge::handle_plus_minus): If
> IFN_ADDC/IFN_SUBC can't be used and rhs1_type is not the limb type
> and is signed, perform both additions or both subtractions in
> unsigned type for the rhs1_type and cast to rhs1_type at the end.
>
> * gcc.dg/torture/bitint-106.c: New test.
>
> --- gcc/gimple-lower-bitint.cc.jj 2026-07-29 10:04:21.350852573 +0200
> +++ gcc/gimple-lower-bitint.cc 2026-07-30 19:24:50.354031989 +0200
> @@ -1287,16 +1287,27 @@ bitint_large_huge::handle_plus_minus (tr
> }
> else
> {
> - tree in = add_cast (rhs1_type, data_in);
> - lhs = make_ssa_name (rhs1_type);
> + /* Always perform the two additions or subtractions in
> + unsigned type, avoid introducing a temporary UB. While
> + the end result of the 2 additions or 2 subtractions in
> + a valid program should not overflow, temporarily it can.
> + See PR126503. */
> + tree utype = unsigned_type_for (rhs1_type);
> + tree in = add_cast (utype, data_in);
> + lhs = make_ssa_name (utype);
> + if (utype != rhs1_type)
> + {
> + rhs1 = add_cast (utype, rhs1);
> + rhs2 = add_cast (utype, rhs2);
> + }
> g = gimple_build_assign (lhs, code, rhs1, rhs2);
> insert_before (g);
> - rhs1 = make_ssa_name (rhs1_type);
> + rhs1 = make_ssa_name (utype);
> g = gimple_build_assign (rhs1, code, lhs, in);
> insert_before (g);
> m_data[m_data_cnt] = NULL_TREE;
> m_data_cnt += 2;
> - return rhs1;
> + return utype != rhs1_type ? add_cast (rhs1_type, rhs1) : rhs1;
> }
> rhs1 = make_ssa_name (m_limb_type);
> g = gimple_build_assign (rhs1, REALPART_EXPR,
> --- gcc/testsuite/gcc.dg/torture/bitint-106.c.jj 2026-07-30
> 19:27:34.671045358 +0200
> +++ gcc/testsuite/gcc.dg/torture/bitint-106.c 2026-07-30
> 19:26:43.798660414 +0200
> @@ -0,0 +1,38 @@
> +/* PR tree-optimization/126503 */
> +/* { dg-do run { target bitint575 } } */
> +
> +[[gnu::noipa]] unsigned _BitInt(400)
> +foo (_BitInt(257) a)
> +{
> + return (unsigned _BitInt(400)) (-a);
> +}
> +
> +[[gnu::noipa]] unsigned _BitInt(300)
> +bar (_BitInt(7) a, unsigned _BitInt(17) b)
> +{
> + _BitInt(257) x = (_BitInt(257)) a + -1;
> + return ~((unsigned _BitInt(300)) x ^ (unsigned _BitInt(300)) b);
> +}
> +
> +[[gnu::noipa]] _BitInt(129)
> +baz (_BitInt(129) x)
> +{
> + return x - 24;
> +}
> +
> +[[gnu::noipa]] int
> +qux (_BitInt(129) x, _BitInt(129) y)
> +{
> + return x == y;
> +}
> +
> +int
> +main ()
> +{
> + if (foo (-1wb) != 1uwb)
> + __builtin_abort ();
> + if (bar (1wb, 3uwb) != (unsigned _BitInt(300)) -4wb)
> + __builtin_abort ();
> + if (!qux (baz (100), 76))
> + __builtin_abort ();
> +}
>
> Jakub
>
