On Tue, 11 Nov 2025, Jakub Jelinek wrote:
> Hi!
>
> Since r11-2238-ge443d8213864ac337c29092d4767224f280d2062 the C++ FE
> emits clobbers like *_1 = {CLOBBER}; where *_1 MEM_REF has some scalar
> type like int for -flifetime-dse={1,2} and most of the compiler manages
> to cope with that.
> If we are very unlucky, we trigger an ICE while trying to regimplify it
> (at least during inlining), as happens with GCC 15.2 on firefox-145.0
> built with LTO+PGO.
> I haven't managed to reduce that to a small testcase that would ICE though,
> the clobber certainly appears in code like
> template <typename T>
> struct S {
> T *p;
> union { char a; T b; };
> static S foo (T *x) { S s; s.p = x; s.b.~T (); return s; }
> ~S ();
> };
>
> void
> bar ()
> {
> int i = 42;
> S <int> s = S <int>::foo (&i);
> }
> but convincing inliner that it should id->regimplify = true; on exactly
> that stmt has been difficult.
>
> The ICE is because we try (in two spots) to regimplify the rhs of the
> gimple_clobber_p stmt if gimple-reg-type type (i.e. the TREE_CLOBBER),
> because it doesn't satisfy the is_gimple_mem_rhs_or_call predicate
> returned by rhs_predicate_for for the MEM_REF lhs. And regimplify it
> by trying to gimplify SSA_NAME = {CLOBBER}; INIT_EXPR and in there reach
> a special case which stores that freshly made SSA_NAME into memory and
> loads it from memory, so uses a SSA_NAME without SSA_NAME_DEF_STMT.
>
> Fixed thusly by saying clobbers are ok even for the gimple-reg-types.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk
> and release branches?
OK.
Thanks,
Richard.
> 2025-11-11 Jakub Jelinek <[email protected]>
>
> PR lto/122620
> * gimplify-me.cc (gimple_regimplify_operands): Don't try to regimplify
> TREE_CLOBBER on rhs of gimple_clobber_p if it has gimple_reg_type.
>
> --- gcc/gimplify-me.cc.jj 2025-10-22 13:30:36.000000000 +0200
> +++ gcc/gimplify-me.cc 2025-11-10 15:32:24.747470202 +0100
> @@ -232,9 +232,13 @@ gimple_regimplify_operands (gimple *stmt
> else if (i == 2
> && gimple_assign_single_p (stmt)
> && num_ops == 2)
> - gimplify_expr (&op, &pre, NULL,
> - rhs_predicate_for (gimple_assign_lhs (stmt)),
> - fb_rvalue);
> + {
> + if (gimple_clobber_p (stmt))
> + continue;
> + gimplify_expr (&op, &pre, NULL,
> + rhs_predicate_for (gimple_assign_lhs (stmt)),
> + fb_rvalue);
> + }
> else if (i == 2 && is_gimple_call (stmt))
> {
> if (TREE_CODE (op) == FUNCTION_DECL)
> @@ -253,8 +257,9 @@ gimple_regimplify_operands (gimple *stmt
> {
> bool need_temp = false;
>
> - if (gimple_assign_single_p (stmt)
> - && num_ops == 2)
> + if (gimple_clobber_p (stmt))
> + ;
> + else if (gimple_assign_single_p (stmt) && num_ops == 2)
> gimplify_expr (gimple_assign_rhs1_ptr (stmt), &pre, NULL,
> rhs_predicate_for (gimple_assign_lhs (stmt)),
> fb_rvalue);
>
> Jakub
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
