ImDrawList: Not using alloca() anymore, lift single polygon size limits. (#5704, #1811)

docs/CHANGELOG.txt

@@ -148,6 +148,7 @@ Other Changes:
   which would result in an abnormal number of vertices created (was slower and more likely to
   asserts with 16-bits ImDrawVtx). (#5720)
 - Fonts: Added GetGlyphRangesGreek() helper for Greek & Coptic glyph range. (#5676, #5727) [@azonenberg]
+- ImDrawList: Not using alloca() anymore, lift single polygon size limits. (#5704, #1811)
 - Platform IME: [Windows] Removed call to ImmAssociateContextEx() leading to freeze on some setups.
   (#2589, #5535, #5264, #4972)
 - Misc: ImGuiKey is now a typed enum, allowing ImGuiKey_XXX symbols to be named in debuggers. (#4921)

imgui.cpp

@@ -4737,6 +4737,7 @@ void ImGui::Shutdown()
         IM_DELETE(g.IO.Fonts);
     }
     g.IO.Fonts = NULL;
+    g.DrawListSharedData.TempBuffer.clear();
 
     // Cleanup of other data are conditional on actually having initialized Dear ImGui.
     if (!g.Initialized)

imgui.h

@@ -23,7 +23,7 @@
 // Library Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals, e.g. '#if IMGUI_VERSION_NUM > 12345')
 #define IMGUI_VERSION               "1.89 WIP"
-#define IMGUI_VERSION_NUM           18828
+#define IMGUI_VERSION_NUM           18829
 #define IMGUI_HAS_TABLE
 
 /*
@@ -2513,7 +2513,7 @@ struct ImDrawList
 
     // [Internal, used while building lists]
     unsigned int            _VtxCurrentIdx;     // [Internal] generally == VtxBuffer.Size unless we are past 64K vertices, in which case this gets reset to 0.
-    const ImDrawListSharedData* _Data;          // Pointer to shared draw data (you can use ImGui::GetDrawListSharedData() to get the one from current ImGui context)
+    ImDrawListSharedData*   _Data;              // Pointer to shared draw data (you can use ImGui::GetDrawListSharedData() to get the one from current ImGui context)
     const char*             _OwnerName;         // Pointer to owner window's name for debugging
     ImDrawVert*             _VtxWritePtr;       // [Internal] point within VtxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
     ImDrawIdx*              _IdxWritePtr;       // [Internal] point within IdxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
@@ -2525,7 +2525,7 @@ struct ImDrawList
     float                   _FringeScale;       // [Internal] anti-alias fringe is scaled by this value, this helps to keep things sharp while zooming at vertex buffer content
 
     // If you want to create ImDrawList instances, pass them ImGui::GetDrawListSharedData() or create and use your own ImDrawListSharedData (so you can use ImDrawList without ImGui)
-    ImDrawList(const ImDrawListSharedData* shared_data) { memset(this, 0, sizeof(*this)); _Data = shared_data; }
+    ImDrawList(ImDrawListSharedData* shared_data) { memset(this, 0, sizeof(*this)); _Data = shared_data; }
 
     ~ImDrawList() { _ClearFreeMemory(); }
     IMGUI_API void  PushClipRect(const ImVec2& clip_rect_min, const ImVec2& clip_rect_max, bool intersect_with_current_clip_rect = false);  // Render-level scissoring. This is passed down to your render function but not used for CPU-side coarse clipping. Prefer using higher-level ImGui::PushClipRect() to affect logic (hit-testing and widget culling)

imgui_draw.cpp

@@ -39,25 +39,12 @@ Index of this file:
 #endif
 
 #include <stdio.h>      // vsnprintf, sscanf, printf
-#if !defined(alloca)
-#if defined(__GLIBC__) || defined(__sun) || defined(__APPLE__) || defined(__NEWLIB__)
-#include <alloca.h>     // alloca (glibc uses <alloca.h>. Note that Cygwin may have _WIN32 defined, so the order matters here)
-#elif defined(_WIN32)
-#include <malloc.h>     // alloca
-#if !defined(alloca)
-#define alloca _alloca  // for clang with MS Codegen
-#endif
-#else
-#include <stdlib.h>     // alloca
-#endif
-#endif
 
 // Visual Studio warnings
 #ifdef _MSC_VER
 #pragma warning (disable: 4127)     // condition expression is constant
 #pragma warning (disable: 4505)     // unreferenced local function has been removed (stb stuff)
 #pragma warning (disable: 4996)     // 'This function or variable may be unsafe': strcpy, strdup, sprintf, vsnprintf, sscanf, fopen
-#pragma warning (disable: 6255)     // [Static Analyzer] _alloca indicates failure by raising a stack overflow exception.  Consider using _malloca instead.
 #pragma warning (disable: 26451)    // [Static Analyzer] Arithmetic overflow : Using operator 'xxx' on a 4 byte value and then casting the result to a 8 byte value. Cast the value to the wider type before calling operator 'xxx' to avoid overflow(io.2).
 #pragma warning (disable: 26812)    // [Static Analyzer] The enum type 'xxx' is unscoped. Prefer 'enum class' over 'enum' (Enum.3). [MSVC Static Analyzer)
 #endif
@@ -67,9 +54,6 @@ Index of this file:
 #if __has_warning("-Wunknown-warning-option")
 #pragma clang diagnostic ignored "-Wunknown-warning-option"         // warning: unknown warning group 'xxx'                      // not all warnings are known by all Clang versions and they tend to be rename-happy.. so ignoring warnings triggers new warnings on some configuration. Great!
 #endif
-#if __has_warning("-Walloca")
-#pragma clang diagnostic ignored "-Walloca"                         // warning: use of function '__builtin_alloca' is discouraged
-#endif
 #pragma clang diagnostic ignored "-Wunknown-pragmas"                // warning: unknown warning group 'xxx'
 #pragma clang diagnostic ignored "-Wold-style-cast"                 // warning: use of old-style cast                            // yes, they are more terse.
 #pragma clang diagnostic ignored "-Wfloat-equal"                    // warning: comparing floating point with == or != is unsafe // storing and comparing against same constants ok.
@@ -753,7 +737,8 @@ void ImDrawList::AddPolyline(const ImVec2* points, const int points_count, ImU32
 
         // Temporary buffer
         // The first <points_count> items are normals at each line point, then after that there are either 2 or 4 temp points for each line point
-        ImVec2* temp_normals = (ImVec2*)alloca(points_count * ((use_texture || !thick_line) ? 3 : 5) * sizeof(ImVec2)); //-V630
+        _Data->TempBuffer.reserve_discard(points_count * ((use_texture || !thick_line) ? 3 : 5));
+        ImVec2* temp_normals = _Data->TempBuffer.Data;
         ImVec2* temp_points = temp_normals + points_count;
 
         // Calculate normals (tangents) for each line segment
@@ -1001,7 +986,8 @@ void ImDrawList::AddConvexPolyFilled(const ImVec2* points, const int points_coun
         }
 
         // Compute normals
-        ImVec2* temp_normals = (ImVec2*)alloca(points_count * sizeof(ImVec2)); //-V630
+        _Data->TempBuffer.reserve_discard(points_count);
+        ImVec2* temp_normals = _Data->TempBuffer.Data;
         for (int i0 = points_count - 1, i1 = 0; i1 < points_count; i0 = i1++)
         {
             const ImVec2& p0 = points[i0];

imgui_internal.h

@@ -734,6 +734,9 @@ struct IMGUI_API ImDrawListSharedData
     ImVec4          ClipRectFullscreen;         // Value for PushClipRectFullscreen()
     ImDrawListFlags InitialFlags;               // Initial flags at the beginning of the frame (it is possible to alter flags on a per-drawlist basis afterwards)
 
+    // [Internal] Temp write buffer
+    ImVector<ImVec2> TempBuffer;
+
     // [Internal] Lookup tables
     ImVec2          ArcFastVtx[IM_DRAWLIST_ARCFAST_TABLE_SIZE]; // Sample points on the quarter of the circle.
     float           ArcFastRadiusCutoff;                        // Cutoff radius after which arc drawing will fallback to slower PathArcTo()