DPC++ device frontend never terminates on imgui_internal.h

[johguenther]

Describe the bug

Just including imgui_internal.h (Dear ImGui v1.92.9b) spins "forever", 100% CPU, memory flat at 54 MB.

compiler
oneAPI 2025.1 / .2 / .3	ok ~1.5 s
v6.2.0	ok 0.9 s
v7.0.0	killed at 300 s
nightly-2026-08-17	killed at 90 s, i.e. not fixed upstream

To reproduce

git clone --depth 1 --branch v1.92.9b https://github.com/ocornut/imgui.git
echo '#include "imgui_internal.h"' > repro.cpp
clang++ -fsycl -fsycl-device-only -fsyntax-only -Iimgui repro.cpp

Device pass only. -fsycl-is-host and plain C++ 0.2 s, preprocessing unaffected.
Up to v7.0.0 one cc1 flag on the host triple suffices, no SYCL headers: clang++ -fsyntax-only -Xclang -fsycl-is-device -Iimgui repro.cpp.
Newer nightlies reject a non-SPIR-V triple there, so use -fsycl -fsycl-device-only for cross-version runs.

Including imgui.h alone is fine. Needs the dense mutually-referential graph of ImGuiContext/ImGuiWindow.

Attaching files failed, minimal self-contained (no #include) repro by reducing the header, ill-formed by design (-ferror-limit=0): 0.1 s v6.2.0 vs 2.0 s v7.0.0, 20x slowdown (not hanging):

typedef signed short ImS16;
typedef unsigned short ImU16;
typedef signed int ImS32;
typedef unsigned int ImU32;
typedef signed long long ImS64;
typedef unsigned long long ImU64;
struct ImDrawChannel;
struct ImDrawCmd;
struct ImDrawData;
struct ImDrawList;
struct ImDrawListSharedData;
struct ImDrawListSplitter;
struct ImDrawVert;
struct ImFont;
struct ImFontAtlas;
struct ImFontAtlasBuilder;
struct ImFontConfig;
struct ImFontGlyph;
struct ImFontGlyphRangesBuilder;
struct ImFontLoader;
struct ImTextureData;
struct ImTextureRect;
struct ImGuiSelectionExternalStorage;
struct ImGuiSelectionRequest;
struct ImGuiSizeCallbackData;
struct ImGuiStorage;
enum ImGuiDir : int;
enum ImGuiKey : int;
enum ImGuiMouseSource : int;
enum ImGuiSortDirection : ImU8;
typedef int ImGuiCol;
typedef int ImGuiListClipperFlags;
typedef unsigned short ImWchar16;
template <typename T> struct ImVector {
  int Size;
  int Capacity;
  T *Data;
  ImVec2 WindowTitleAlign;
  ImGuiDir WindowMenuButtonPosition;
  float ChildRounding;
  float ChildBorderSize;
  float PopupRounding;
  ImVec2 DisplayWindowPadding;
  ImVec2 DisplaySafeAreaPadding;
  float MouseCursorScale;
  bool AntiAliasedLines;
  bool AntiAliasedLinesUseTex;
  bool AntiAliasedFill;
  float CurveTessellationTol;
  float CircleTessellationMaxError;
  ImVec4 Colors[ImGuiCol_COUNT];
  float HoverStationaryDelay;
  float HoverDelayShort;
  float HoverDelayNormal;
  ImGuiHoveredFlags HoverFlagsForTooltipMouse;
  ImGuiHoveredFlags HoverFlagsForTooltipNav;
  ImFontAtlas *Fonts;
  ImFont *FontDefault;
  void SetStatus(ImTextureStatus status) {
    Status = status;
    if (status == ImTextureStatus_Destroyed && !WantDestroyNextFrame &&
        Pixels != nullptr)
      Status = ImTextureStatus_WantCreate;
  }
  ImFont *DstFont;
  const ImFontLoader *FontLoader;
  void *FontLoaderData;
  ImFontAtlasFlags_NoBakedLines = 1 << 2,
};
struct ImFontAtlas {
  ImFontAtlas();
  ~ImFontAtlas();
  ImFont *AddFont(const ImFontConfig *font_cfg);
  ImFont *AddFontDefault(const ImFontConfig *font_cfg = 0);
  ImVector<ImTextureData *> TexList;
  bool RendererHasTextures;
  bool TexIsBuilt;
  ImVec2 TexUvWhitePixel;
  ImVector<ImFont *> Fonts;
  ImVector<ImFontConfig> Sources;
  ImVec4 TexUvLines[(32) + 1];
  int TexNextUniqueID;
  int FontNextUniqueID;
  ImVector<ImDrawListSharedData *> DrawListSharedDatas;
  ImFontAtlasBuilder *Builder;
  const ImFontLoader *FontLoader;
  const char *FontLoaderName;
  void *FontLoaderData;
  int RefCount;
  ImGuiContext *OwnerContext;
  ImVector<float> IndexAdvanceX;
  float FallbackAdvanceX;
  float Size;
  float RasterizerDensity;
  ImVector<ImU16> IndexLookup;
  ImVector<ImFontGlyph> Glyphs;
  int FallbackGlyphIndex;
  float Ascent, Descent;
  unsigned int MetricsTotalSurface : 26;
  unsigned int WantDestroy : 1;
  unsigned int LoadNoFallback : 1;
  unsigned int LoadNoRenderOnLayout : 1;
  int LastUsedFrame;
  ImGuiID BakedId;
  ImFont *OwnerFont;
};
struct ImFont {
  ImFontBaked *LastBaked;
  ImFontAtlas *OwnerAtlas;
  ImFontFlags Flags;
  float CurrentRasterizerDensity;
  ImVector<ImFontConfig *> Sources;
}
}
;
struct ImRect {
  ImVec2 Min;
  ImVec2 Max;
  ImVector<ImVec2> TempBuffer;
  ImVector<ImDrawList *> DrawLists;
  ImVector<ImFontAtlas *> FontAtlases;
  ImFont *Font;
  ImVector<float> ItemWidthStack;
  ImVector<float> TextWrapPosStack;
};
struct ImGuiWindow {
  ImGuiWindowFlags Flags;
  ImRect OuterRectClipped;
  ImRect InnerRect;
  ImRect InnerClipRect;
  ImRect WorkRect;
  ImRect ParentWorkRect;
  ImRect ClipRect;
  ImGuiWindow *ParentWindow;
  ImGuiWindow *ParentWindowInBeginStack;
  ImGuiWindow *RootWindow;
  ImGuiWindow *RootWindowPopupTree;
};
ImGuiWindow *OuterWindow;

Environment

• OS: independent
• Target device and vendor: N/A, frontend only
• DPC++ version: bisected

  last good: nightly-2025-09-24   3276ddd75dae5563f336737daddcbb151484702b
  first bad: nightly-2025-09-30   0ff1a5c2b4e4bc56799ec2dd17a89c3c57608890
  

Additional context

Hypothesis from Claude:

checkSYCLType() in clang/lib/Sema/SemaSYCL.cpp, from SemaSYCL::checkSYCLDeviceVarDecl, device mode only. Walks all fields, follows pointers/arrays into the pointee's fields. Visited passed by value => memoizes per DFS path, not globally => enumerates simple paths instead of visiting each type once. See #17313 (open, wants checkSYCLType merged into Sema::checkTypeSupport); mentions no perf impact, but same function.

Exponential in both versions: on v6.2.0, nesting struct W { ImGuiContext* a; ImGuiContext* b; };
N deep multiplies time by ~1.8/level (0.7, 0.9, 1.3, 2.0, 3.7 s). So not a new loop — something in the Sept 2025 window weakened dedup enough to push ImGui's graph past any usable threshold;
v6.2.0 needs ~9 extra doublings to reach v7.0.0.