[Issue]: [Bug][Linux] flm serve crash: C++ assertion failure '!this->empty()' in std::vector<logits_t>::back()

Problem Description

Crashed Thread Stack Trace:

1 Thread 1 (Thread 0x7f92069fd6c0 (LWP 3577901)):
2 #0 __pthread_kill_implementation (threadid=, signo=signo@entry=6, no_tid=no_tid@entry=0) at
pthread_kill.c:44
3 #1 0x00007f934609a243 in __pthread_kill_internal (threadid=, signo=6) at pthread_kill.c:89
4 #2 0x00007f934603e5d0 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
5 #3 0x00007f9346025685 in __GI_abort () at abort.c:77
6 #4 0x00007f934649d5bd in std::__glibcxx_assert_fail (file=, line=, function=, condition=) at ../../../../../gcc/libstdc++-v3/src/c++11/assert_fail.cc:41
7 #5 0x0000562ef5ae03ae in ?? ()
8 #6 0x0000562ef5d3bd67 in ?? ()

Root Cause & Technical Analysis (GDB Core Extraction):
By examining the assembly and register values inside flm right before the abort call (std::__glibcxx_assert_fail), we
extracted the RIP-relative string parameters:

• Assertion File: /usr/include/c++/16/bits/stl_vector.h (Line 1360)
• Assertion Function:

1 constexpr std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::back() [with _Tp = logits_t; _Alloc =
std::allocator<logits_t>; reference = logits_t&]

• Failed Condition: !this->empty()

The program counter at 0x0000562ef5ae03ae is preceded by the assertion instruction block:

1 0x0000562ef5ae038e: lea 0x6a5b31(%rip),%rcx # 0x562ef6185ec6 -> "!this->empty()"
2 0x0000562ef5ae0395: lea 0x6beb34(%rip),%rdx # 0x562ef619eed0 -> "std::vector<logits_t>::back()"
3 0x0000562ef5ae039c: mov $0x550,%esi # 0x550 = Line 1360
4 0x0000562ef5ae03a1: lea 0x6b3708(%rip),%rdi # 0x562ef6193ab0 -> "/usr/include/c++/16/bits/stl_vector.h"
5 0x0000562ef5ae03a8: call *0x94e75a(%rip) # Calls __glibcxx_assert_fail

This proves that flm called logits.back() when logits had a size of 0.

Environment & Package Details:

• OS: Arch Linux (Kernel 6.x / AMD Ryzen AI NPU configuration)
• Compiler/Libc: GCC 16 / glibc 2.40 / libstdc++ 16.x (compiled with standard -D_GLIBCXX_ASSERTIONS)
• FastFlowLM Version: fastflowlm 1.0.4-1
• Model in use: qwen3-it:4b (Qwen3-4B-Instruct-2507-NPU2)
• Hardware: AMD Ryzen NPU (managed via /usr/lib/libxrt_driver_xdna.so.2)

Proposed Fix:
Add a safety check to ensure that the logits vector is populated before fetching the trailing element:

1 if (!logits.empty()) {
2 auto last_logit = logits.back();
3 // ... process logit
4 } else {
5 // Handle empty logits list gracefully or return error code
6 }

Filed by Gemini via Gemini CLI.

Operating System

Omarchy 4.0.3-1

CPU

AMD Ryzen AI 5 340 w/ Radeon 840M

GPU

iGPU: AMD Radeon 840M (Krackan, 1002:1114) NPU: NPU Krackan 1 [0000:c2:00.1] (used by FastFlowLM)

ROCm Version

ROCm not installed. FastFlowLM uses XRT 2.21.75 + NPU firmware 1.1.2.64 (not the ROCm GPU stack).

Installation Method

Other (describe in Additional Information)

Installed ROCm Packages / Versions

ROCm Component

No response

Steps to Reproduce

No response

(Optional for Linux users) Output of rocminfo --support

Paste output here

Additional Information

No response

Start with the flm serve execution path that reaches std::vector<logits_t>::back(), using the supplied GDB assertion and stack trace to locate the call site. Reproduce on the reported AMD Ryzen AI NPU environment and determine how an empty logits vector is reached; done means the request no longer aborts when logits are empty and the failure is handled gracefully.