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Updated to be more informative, based on draft blog post.
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‎docs/build/reference/qspectre-load.md‎

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---
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title: "/Qspectre-load"
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description: "Describes the Microsoft C/C++ compiler (MSVC) /Qspectre-load option."
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ms.date: "01/08/2020"
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ms.date: "01/28/2020"
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helpviewer_keywords: ["/Qspectre-load"]
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---
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# /Qspectre-load
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## Remarks
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The **/Qspectre-load** 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.
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**/Qspectre-load** causes the compiler to detect loads from memory, and insert serializing instructions after them. Control flow instructions that load memory, including `RET` and `CALL`, 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 control flow 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 non-destructively before inserting an LFENCE, as shown here:
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```asm
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xor rbx, [rax]
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xor rbx, [rax] ; force a load of [rax]
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lfence ; followed by an LFENCE
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jmp [rax]
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```
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**/Qspectre-load** causes the compiler to detect loads from memory, and insert serializing instructions after them. Control flow instructions that load memory, including `ret` and `call`, are split into a load and a control flow transfer to ensure the load is protected. There are cases where the compiler can't split control flow instructions, such as the `jmp` instruction, so it uses an alternate mitigation technique.
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Because **/Qspectre-load** stops speculation of all loads, the performance impact is high. The mitigation isn't appropriate everywhere. If there are performance critical blocks of code that do not require protection, you can disable these mitigations by using `__declspec(spectre(nomitigation))`. For more information, see [__declspec spectre](../../cpp/spectre.md).
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The **/Qspectre-load** option is off by default, and supports all optimization levels.
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The **/Qspectre-load** 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.
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### To set this compiler option in the Visual Studio development environment
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1. Open the project's **Property Pages** dialog box. For details, see [Set C++ compiler and build properties in Visual Studio](../working-with-project-properties.md).
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## See also
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[/Q Options (Low-Level Operations)](q-options-low-level-operations.md)\
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[MSVC Compiler Options](compiler-options.md)\
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[MSVC Compiler Command-Line Syntax](compiler-command-line-syntax.md)
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[/Q options (Low-Level Operations)](q-options-low-level-operations.md)\
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[MSVC compiler options](compiler-options.md)\
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[MSVC compiler command-line syntax](compiler-command-line-syntax.md)

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