Edit

Share via

/Qspectre-load-cf

  • Article

Specifies compiler generation of serializing instructions for every control-flow instruction that contains a load. This option performs a subset of the mitigations done by the /Qspectre-load option.

Syntax

/Qspectre-load-cf

Remarks

/Qspectre-load-cf causes the compiler to detect JMP, RET, and CALL control-flow instructions that load from memory, and to insert serializing instructions after the load. Where possible, these instructions are split into a load and a control flow transfer. The load is followed by an LFENCE to ensure the load is protected. There are cases where the compiler can't split instructions, such as the JMP instruction, so it uses an alternate mitigation technique. For example, the compiler mitigates jmp [rax] by adding instructions to load the target nondestructively before inserting an LFENCE, as shown here:

    xor rbx, [rax]
    xor rbx, [rax]  ; force a load of [rax]
    lfence          ; followed by an LFENCE
    jmp [rax]

Because /Qspectre-load-cf stops speculation of all loads in control-flow instructions, the performance impact is high. The mitigation isn't appropriate everywhere. If there are performance critical blocks of code that don't require protection, you can disable these mitigations by using __declspec(spectre(nomitigation)).

The /Qspectre-load-cf option is off by default, and supports all optimization levels.

The /Qspectre-load-cf option is available in Visual Studio 2019 version 16.5 and later. This option is only available in compilers that target x86 and x64 processors. It's not available in compilers that target ARM processors.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Select the Configuration Properties > C/C++ > Code Generation property page.

  3. Select a new value for the Spectre Mitigation property. Choose OK to apply the change.

To set this compiler option programmatically

See also

/Qspectre
/Qspectre-jmp
/Qspectre-load
/Q options (Low-level operations)
MSVC compiler options
MSVC compiler command-line syntax