[ARM/ARM64] C# interop broken due to HVA ABI

[JunaMeinhold]

On ARM/ARM64, the current Hexa.NET.ImGui bindings break because C# cannot handle homogeneous vector aggregates (HVAs) correctly. Functions that take or return ImVec2, ImVec4, or similar types fail at runtime due to ABI mismatches.

Root Cause:

• ARM64 ABI passes HVAs in SIMD registers.
• C# P/Invoke does not support HVAs, leading to corrupted arguments/returns.
• Works on x86/x64 because ABI rules differ.

Proposed Solution:
Develop a C++ → C generator for the bindings:

1. Automatically generate C ABI-safe wrapper functions from the ImGui C++ API.
2. Replace vector-type parameters/returns with scalar equivalents or plain structs with const ptr.
3. Expose only C-style functions to C#, ensuring ABI compatibility across ARM/ARM64 and x86/x64.

Benefits:

• Eliminates manual binding errors.
• Ensures ARM64 compatibility.
• Keeps API surface consistent for managed consumers.

Action Items:

• Prototype CPP→C generator for a subset of ImGui functions. HexaGen#29
• Replace existing vector-type interop in Hexa.NET.ImGui.
• Validate ARM64 runtime behavior with generated wrappers.

Notes:

• This approach is preferable to attempting direct P/Invoke of C++ vectors.
• Could be extended in the future for other C++ libraries with similar issues.

[JunaMeinhold]

Update: There is a workaround available on windows, by force building for x86 and using the build-in x86 emulation.

[sonoro1234]

Is that related? cimgui/cimgui#298
Would be the last message a solution?

[JunaMeinhold]

Hi, the last message would be just a workaround but not a solution, a real solution would be to change the c api

from
ImVec2 vec
to
const ImVec2* vec

And yeah the issue is related

[JunaMeinhold]

I have a Surface Pro with an arm64 CPU, I could help testing if you want.

[sonoro1234]

Hi, the last message would be just a workaround but not a solution, a real solution would be to change the c api

from ImVec2 vec to const ImVec2* vec

And yeah the issue is related

Could you elaborate with a C function example?
igGetColorU32_Vec4 ?

I have a Surface Pro with an arm64 CPU, I could help testing if you want.

Sure!!

Did you noticed it only happens with MSVC and no in mingw-w64?

[JunaMeinhold]

I will do some more tests today and try both clang and gcc for Windows and report back later

[JunaMeinhold]

For additional context
https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170

[JunaMeinhold]

Okay small update, could you give me the binary from mingw, because i think that the output dll is not arm64 but rather x86_64 which is just the workaround i mentioned, but it comes with an performance impact, because windows emulates (jits) it to arm64 which adds overhead.

[sonoro1234]

I dont have it!!
It was checked in the related cimgui issue by the person posting the issue.

[JunaMeinhold]

Ah okay sorry.
I found out that doing this resolves the issue

[sonoro1234]

I am doing a list with all structs potentially not ABI compatible used as returns or args.

[sonoro1234]

This the list of structs with constructor used as args to functions:

[1]="ImColor*"
        [2]="ImDrawCmd*"
        [3]="ImDrawData*"
        [4]="ImDrawDataBuilder*"
        [5]="ImDrawList*"
        [6]="ImDrawListSharedData*"
        [7]="ImDrawListSplitter*"
        [8]="ImFont*"
        [9]="ImFontAtlas*"
        [10]="ImFontAtlasBuilder*"
        [11]="ImFontAtlasRect*"
        [12]="ImFontBaked*"
        [13]="ImFontConfig*"
        [14]="ImFontGlyph*"
        [15]="ImFontGlyphRangesBuilder*"
        [16]="ImFontLoader*"
        [17]="ImGuiBoxSelectState*"
        [18]="ImGuiComboPreviewData*"
        [19]="ImGuiContext*"
        [20]="ImGuiContextHook*"
        [21]="ImGuiDebugAllocInfo*"
        [22]="ImGuiDockContext*"
        [23]="ImGuiDockNode*"
        [24]="ImGuiErrorRecoveryState*"
        [25]="ImGuiIDStackTool*"
        [26]="ImGuiIO*"
        [27]="ImGuiInputEvent*"
        [28]="ImGuiInputTextCallbackData*"
        [29]="ImGuiInputTextDeactivatedState*"
        [30]="ImGuiInputTextState*"
        [31]="ImGuiKeyOwnerData*"
        [32]="ImGuiKeyRoutingData*"
        [33]="ImGuiKeyRoutingTable*"
        [34]="ImGuiLastItemData*"
        [35]="ImGuiListClipper*"
        [36]="ImGuiListClipperData*"
        [37]="ImGuiMenuColumns*"
        [38]="ImGuiMultiSelectState*"
        [39]="ImGuiMultiSelectTempData*"
        [40]="ImGuiNavItemData*"
        [41]="ImGuiNextItemData*"
        [42]="ImGuiNextWindowData*"
        [43]="ImGuiOldColumnData*"
        [44]="ImGuiOldColumns*"
        [45]="ImGuiOnceUponAFrame*"
        [46]="ImGuiPayload*"
        [47]="ImGuiPlatformIO*"
        [48]="ImGuiPlatformImeData*"
        [49]="ImGuiPlatformMonitor*"
        [50]="ImGuiPopupData*"
        [51]="ImGuiPtrOrIndex*"
        [52]="ImGuiSelectionBasicStorage*"
        [53]="ImGuiSelectionExternalStorage*"
        [54]="ImGuiSettingsHandler*"
        [55]="ImGuiStackLevelInfo*"
        [56]="ImGuiStoragePair*"
        [57]="ImGuiStyle*"
        [58]="ImGuiStyleMod*"
        [59]="ImGuiTabBar*"
        [60]="ImGuiTabItem*"
        [61]="ImGuiTable*"
        [62]="ImGuiTableColumn*"
        [63]="ImGuiTableColumnSettings*"
        [64]="ImGuiTableColumnSortSpecs*"
        [65]="ImGuiTableInstanceData*"
        [66]="ImGuiTableSettings*"
        [67]="ImGuiTableSortSpecs*"
        [68]="ImGuiTableTempData*"
        [69]="ImGuiTextBuffer*"
        [70]="ImGuiTextFilter*"
        [71]="ImGuiTextRange*"
        [72]="ImGuiTypingSelectState*"
        [73]="ImGuiViewport*"
        [74]="ImGuiViewportP*"
        [75]="ImGuiWindow*"
        [76]="ImGuiWindowClass*"
        [77]="ImGuiWindowSettings*"
        [78]="ImRect*"
        [79]="ImTextureData*"
        [80]="ImTextureRef"
        [81]="ImTextureRef*"
        [82]="ImVec1*"
        [83]="ImVec2"
        [84]="ImVec2*"
        [85]="ImVec2i*"
        [86]="ImVec2ih*"
        [87]="ImVec4*"
        [88]="const ImFontConfig*"
        [89]="const ImFontLoader*"
        [90]="const ImRect"
        [91]="const ImVec2"
        [92]="const ImVec2*"
        [93]="const ImVec4"
        [94]="const ImVec4*"

Only five are not pointers but if for example we use ImVec2* and is pointer to cimgui::ImVec2 the layout would be not OK also

so in dear_bindings there is:

CIMGUI_API const char*  cimgui::ImFont_GetDebugName(const cimgui::ImFont* self)
{
    return reinterpret_cast<const ::ImFont*>(self)->GetDebugName();
}

but reinterpret_cast is not magic and does not conversion so it seems no useful?

[JunaMeinhold]

I would imagine that passing pointers is totally fine, because when i used ScaleAllSizes(ImStyle*) no errors occurred, and they just redefine cpp classes as c structs and that's probably why they use reinterpret_cast.

like i did here

[sonoro1234]

like i did here

Wrong code?
You mean that ImStyle* used in an ARM64 MSVC compiled works correctly?
ImVec2* works also?