ActivationOpNoInfTest.Softsign fails on x86_64 (AVX/AVX2) due to reciprocal underflow in Eigen

[mudream4869]

Describe the issue

We observed a numerical discrepancy in the Softsign operator when using AVX/AVX2 vectorized paths in recent versions (v1.21+). For extreme input values (e.g., FLT_MAX), the output underflows to 0.0 instead of saturating to ~1.0.

This behavior currently causes the bundled ActivationOpNoInfTest.Softsign to fail on x86_64 platforms.

To reproduce

Build:

./build.sh --config RelWithDebInfo --build_shared_lib --parallel --compile_no_warning_as_error --skip_submodule_sync --cmake_extra_defines onnxruntime_USE_AVX=ON

Unittest Output

[ RUN      ] ActivationOpNoInfTest.Softsign
.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/checkers.cc:471: Failure
The difference between cur_expected[i] and cur_actual[i] is 1, which exceeds tolerance, where
cur_expected[i] evaluates to 1,
cur_actual[i] evaluates to 0, and
tolerance evaluates to 0.00010999999358318746.
i:6
Google Test trace:
.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/checkers.cc:649: provider type: CPUExecutionProvider
.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/base_tester.cc:898: registered execution providers: CPUExecutionProvider

.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/checkers.cc:471: Failure
The difference between cur_expected[i] and cur_actual[i] is 1, which exceeds tolerance, where
cur_expected[i] evaluates to -1,
cur_actual[i] evaluates to -0, and
tolerance evaluates to 0.00010999999358318746.
i:7
Google Test trace:
.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/checkers.cc:649: provider type: CPUExecutionProvider
.../onnxruntime-1.24.4/onnxruntime/test/unittest_util/base_tester.cc:898: registered execution providers: CPUExecutionProvider

[  FAILED  ] ActivationOpNoInfTest.Softsign (1 ms)

Urgency

No response

Platform

Linux

OS Version

24.04

ONNX Runtime Installation

Built from Source

ONNX Runtime Version or Commit ID

v1.21 ~ v1.24.4

ONNX Runtime API

C++

Architecture

X64

Execution Provider

Default CPU

Execution Provider Library Version

No response