clang-cl --target=arm64-pc-windows-msvc doesn't support __umulh needed by WinSDK 10.0.20348.0

[StephanTLavavej]

Bugzilla Link	51128
Resolution	FIXED
Resolved on	Oct 11, 2021 20:29
Version	12.0
OS	Windows NT
Blocks	#51489
CC	@dwblaikie,@mstorsjo,@nico,@rnk,@tstellar
Fixed by commit(s)	cc3affd ddc49d0

Extended Description

Here's how I reproed this (there may be a simpler way). Install VS 2022 17.0 Preview 2. After choosing the "Desktop development with C++" workload, customize it:

• Select the "C++ Clang tools for Windows" (containing Clang 12)
• Select the "MSVC v142 - VS 2019 C++ ARM64 build tools (Latest)" (slightly misnamed; this is the VS 2022 compiler)
• Unselect the default WinSDK 10.0.19041.0
• Select the latest "Windows 10 SDK (10.0.20348.0)"

This setup (Clang 12 + latest WinSDK) works for x86, x64, and ARM (32-bit) in the microsoft/STL test suite; however, we encountered an incompatibility on ARM64 due to the new WinSDK attempting to use the __umulh intrinsic, which works for MSVC but not Clang 12. Self-contained test case, starting from a clean command prompt:

C:\Temp>"C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Auxiliary\Build\vcvarsall.bat" x64_arm64

---

** Visual Studio 2022 Developer Command Prompt v17.0.0-pre.2.0
** Copyright (c) 2021 Microsoft Corporation

---

[vcvarsall.bat] Environment initialized for: 'x64_arm64'

C:\Temp>type meow.cpp
#include <Windows.h>

C:\Temp>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.30.30401 for ARM64
Copyright (C) Microsoft Corporation. All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

C:\Temp>cl /EHsc /nologo /W4 /c meow.cpp
meow.cpp

C:\Temp>clang-cl --target=arm64-pc-windows-msvc --version
clang version 12.0.0
Target: arm64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\Llvm\x64\bin

C:\Temp>clang-cl --target=arm64-pc-windows-msvc /EHsc /nologo /W4 /c meow.cpp
In file included from meow.cpp:1:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\Windows.h:171:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\shared\windef.h:24:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\shared\minwindef.h:182:
C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\winnt.h(6370,20): error: use of undeclared identifier
'__umulh'
*HighProduct = UnsignedMultiplyHigh(Multiplier, Multiplicand);
^
C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um\winnt.h(6236,30): note: expanded from macro
'UnsignedMultiplyHigh'
#define UnsignedMultiplyHigh __umulh
^
1 error generated.

[mstorsjo]

• Unselect the default WinSDK 10.0.19041.0
• Select the latest "Windows 10 SDK (10.0.20348.0)"

Thanks for the report!

The latest SDK I happen to have handy is 10.0.22000.0, and that one happens to have this block of code:

---8<---
// Most ARM64/AArch64 intrinsics available in MSVC are not currently available in other compilers.
// clang-cl defines _M_ARM64, so we need a better escape hatch to avoid using these intrinsics,
// as well as allow us to re-enable them in the event clang-cl starts supporting the msvc intrinsics.
#if !defined(ARM64_MULT_INTRINSICS_SUPPORTED)
#if defined(_MSC_VER) && !defined(clang)
#define ARM64_MULT_INTRINSICS_SUPPORTED 1
#else
#define ARM64_MULT_INTRINSICS_SUPPORTED 0
#endif
#endif
---8<---

In that version, this issue doesn't happen (as the actual use of __umulh is ifdeffed out), but if I enable the use of these intrinsics on Clang, I do get the error you're describing. I take it that the 10.0.20348.0 SDK has removed this workaround?

(Side note, I guess we should dig through the other MSVC headers to see what other intrinsics that might be worthwhile to implement, as the comment seems to imply that there's lots of ones that we're missing.)

In any case, it should be pretty straightforward to add support for these intrinsics, I've got a WIP locally that seems to work; I'll try to brush it up and add tests to make it submittable.

[mstorsjo]

I posted an initial patch to implement this in https://reviews.llvm.org/D106721.

[mstorsjo]

Pushed cc3affd which implements the __mulh and __umulh builtins for MSVC/arm64 mode.

The patch applies cleanly on the release/13.x branch, so if it's not seen as too risky (I think it's quite safe) I think it'd be good to pull this into the upcoming release too.

[tstellar]

Merged: ddc49d0

[tstellar]

mentioned in issue #51489