https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125526
Bug ID: 125526
Summary: asm goto + thread_local "m" input + noreturn branch:
wrong code (-O1), ICE in calc_dfs_tree (-O2)
Product: gcc
Version: 16.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: agri at akamo dot info
Target Milestone: ---
Created attachment 64599
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=64599&action=edit
reproducing program
Summary:
A specific combination of asm volatile goto features causes wrong code
generation at -O1 and an ICE at -O2 on x86-64.
Trigger requires all five of:
1. Early `if (!p) return false` before any stack allocation
2. thread_local variable as "m" asm input (plain global does not trigger)
3. asm volatile goto with "+m" output and conditional jump to a label
4. if (asm_output == 0) noreturn_fn() after the asm
5. Identical expression on both the fall-through and the asm-goto target
paths
Observed behavior:
GCC 14.2 -O1: wrong code — lab_abort label merged with early-return label
(before prologue); `ret` pops local variable instead of return
address → segfault at runtime
GCC 16.1 -O1: correct output, no crash
GCC 16.1 -O2: ICE — internal compiler error in calc_dfs_tree,
dominance.cc:458
Expected behavior:
Compiles and runs correctly at all optimization levels (clang 22.1.0 -O2:
correct)
