ARM Cortex-M55 Helium Intrinsic vdupq_x_n_f32 Predication Issue

Incorrect Behavior of vdupq_x_n_f32 with False Predication Flags

The issue revolves around the unexpected behavior of the ARM Cortex-M55 Helium intrinsic vdupq_x_n_f32 when used with false predication flags. Specifically, the intrinsic vdupq_x_n_f32 is designed to duplicate a scalar value into a vector register while applying predication. However, in the observed scenario, the intrinsic overwrites the entire vector register with the scalar value, even when the predication flags are false. This behavior contradicts the expected outcome where only the lanes corresponding to true predication flags should be modified, while the lanes corresponding to false predication flags should remain unchanged or be set to an undefined value.

The problem manifests in the following code snippet:

mve_pred16_t p_0;
float32x4_t TTT_f4;
TTT_f4 = vdupq_n_f32(5.17);
p_0 = vcmpltq_n_f32(TTT_f4, 0);
TTT_f4 = vdupq_x_n_f32(12.69, p_0);

In this code, TTT_f4 is initialized with the value 5.17 across all lanes. The predication flag p_0 is then set based on the comparison of TTT_f4 with 0. Since 5.17 is not less than 0, all predication flags in p_0 are false. The subsequent call to vdupq_x_n_f32 should, in theory, leave the lanes of TTT_f4 corresponding to false predication flags unchanged. However, the observed behavior is that TTT_f4 is entirely overwritten with 12.69, which is inconsistent with the expected behavior of predication.

Misunderstanding of "_x" Predication Type and Undefined Behavior

The root cause of this issue lies in a misunderstanding of the predication types supported by ARM Helium intrinsics, specifically the distinction between "_x" (don’t care) and "_m" (merging) predication types. The intrinsic vdupq_x_n_f32 uses the "_x" predication type, which is defined as follows:

• "_x" Predication Type (Don’t Care): For lanes where the predication flag is false, the resulting value is undefined. This means that the compiler or hardware is free to assign any value to these lanes, including the possibility of overwriting them with the scalar value being duplicated.

In the provided code, since all predication flags in p_0 are false (because 5.17 is not less than 0), the intrinsic vdupq_x_n_f32 is allowed to assign undefined values to all lanes of TTT_f4. In this case, the undefined behavior resulted in all lanes being overwritten with 12.69.

The confusion arises from the expectation that false predication flags would preserve the original values in the vector register. However, this expectation is only valid for the "_m" (merging) predication type, not the "_x" (don’t care) type. The "_m" predication type explicitly preserves the values of false predicated lanes by merging them with the original vector.

Correct Usage of vdupq_m_n_f32 for Merging Predication

To achieve the desired behavior where false predicated lanes retain their original values, the correct intrinsic to use is vdupq_m_n_f32, which employs the "_m" (merging) predication type. The "_m" predication type ensures that lanes corresponding to false predication flags are filled with values from an inactive vector, typically the original vector being modified.

The corrected code using vdupq_m_n_f32 is as follows:

mve_pred16_t p_0;
float32x4_t TTT_f4;
TTT_f4 = vdupq_n_f32(5.17);
p_0 = vcmpltq_n_f32(TTT_f4, 0.0);
TTT_f4 = vdupq_m_n_f32(TTT_f4, 12.69, p_0);

In this corrected version, vdupq_m_n_f32 is used instead of vdupq_x_n_f32. The intrinsic vdupq_m_n_f32 takes three arguments: the original vector TTT_f4, the scalar value 12.69, and the predication flags p_0. When the predication flags are false, the intrinsic merges the original values from TTT_f4 into the result, preserving the values in the false predicated lanes.

To further illustrate the behavior, consider the following scenarios:

1. All Predication Flags False:

   • Original vector: TTT_f4 = [5.17, 5.17, 5.17, 5.17]
   • Predication flags: p_0 = [false, false, false, false]
   • Result: TTT_f4 = [5.17, 5.17, 5.17, 5.17] (unchanged)

2. All Predication Flags True:

   • Original vector: TTT_f4 = [5.17, 5.17, 5.17, 5.17]
   • Predication flags: p_0 = [true, true, true, true]
   • Result: TTT_f4 = [12.69, 12.69, 12.69, 12.69] (all lanes overwritten)

3. Mixed Predication Flags:

   • Original vector: TTT_f4 = [5.17, 5.17, 5.17, 5.17]
   • Predication flags: p_0 = [true, false, true, false]
   • Result: TTT_f4 = [12.69, 5.17, 12.69, 5.17] (only true predicated lanes overwritten)

This behavior aligns with the intended use of predication in ARM Helium intrinsics, where the "_m" predication type provides a predictable and controlled way to modify vector lanes based on predication flags.

Additional Considerations and Best Practices

When working with ARM Helium intrinsics, it is crucial to understand the differences between predication types and their implications on vector operations. The following table summarizes the key differences between "_x" and "_m" predication types:

Predication Type	Description	Behavior for False Predicated Lanes
"_x" (Don’t Care)	False predicated lanes are assigned undefined values.	Undefined (may overwrite with scalar value)
"_m" (Merging)	False predicated lanes retain their original values from the input vector.	Preserves original values

To avoid similar issues, consider the following best practices:

1. Choose the Correct Predication Type:

   • Use "_m" predication when you need to preserve the values of false predicated lanes.
   • Use "_x" predication only when the values of false predicated lanes are irrelevant.

2. Verify Predication Flags:

   • Ensure that the predication flags are correctly set based on the intended comparison or condition.
   • Use appropriate comparison intrinsics (e.g., vcmpltq_n_f32, vcmpgtq_n_f32) to generate predication flags.

3. Inspect Assembly Code:

   • When debugging, inspect the generated assembly code to verify that the compiler is correctly implementing the predication logic.
   • Look for instructions that correspond to the predication type and ensure they align with your expectations.

4. Refer to Documentation:

   • Consult the ARM Helium Programmer’s Guide and intrinsic documentation for detailed information on predication types and their usage.
   • Pay attention to the specific behavior of each intrinsic and its predication variants.

By following these best practices and understanding the nuances of predication types, developers can effectively leverage ARM Helium intrinsics to optimize performance and achieve the desired behavior in their embedded systems applications.