[cfe-dev] invalid instruction mnemonic with Clang-Cl and FA flag

Gaier, Bjoern via cfe-dev cfe-dev at lists.llvm.org
Wed Mar 24 04:05:59 PDT 2021

Hello Clang-Devs,

I run into a strange error with Clang-Cl - I hope I can explain it. We use Clang-Cl via the LLVM extension in Visual Studio and use it to compile our source code. One of the flags we set for the compiler is "/FA" which gives us assembly output. Everything fine so far, no problems.

I then add a simple copy loop to our code, that is copying int32 values from one address to another one. When compiling this construct Clang-Cl suddenly exited with: "error: invalid instruction mnemonic 'movsdi'".
However, when I remove the "/FA" flag everything compiles fine and runs fine in the end... Strangely, I couldn't figure out what the "movsdi" mnemonic is...

The Assembly code looks like this:
.LBB15_31:                              #   in Loop: Header=BB15_17 Depth=1
                mov       ecx, 32
                mov       edx, 4
                call         calloc
                mov       qword ptr [r12 + rbp + 96], rax
                mov       qword ptr [r12 + rbp + 88], rax
                mov       ecx, 32
                mov       rdi, rax
                rep movsdi es:[rdi], [rsi]
                jmp        .LBB15_33

My Clang-Cl version is:
clang version 11.0.0 (https://github.com/llvm/llvm-project.git 176249bd6732a8044d457092ed932768724a6f06)
Target: x86_64-pc-windows-msvc
Thread model: posix

The target is the same as in the version (x86_64-pc-windows-msvc).

Does anyone know what is going on?

Kind greetings
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