Replace variable-length-arrays with alloca()

Sadly, we can't have nice things that C99 has had for 24 years.
pioneerspacesim · Nov 22, 2023 · 6ffcfcf · 6ffcfcf
1 parent fc36c6a
commit 6ffcfcf
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Showing 2 changed files with 14 additions and 2 deletions.
diff --git a/src/collider/BVHTree.cpp b/src/collider/BVHTree.cpp
@@ -6,6 +6,7 @@
 #include "Aabb.h"
 #include "MathUtil.h"
 #include "core/Log.h"
+#include "core/macros.h"
 #include "profiler/Profiler.h"
 
 const int MAX_SPLITPOS_RETRIES = 15;
@@ -326,7 +327,7 @@ void SingleBVHTree::ComputeOverlap(uint32_t nodeId, const AABBd &nodeAabb, std::
 	PROFILE_SCOPED()
 
 	int32_t stackLevel = 0;
-	uint32_t stack[m_treeHeight + 1];
+	uint32_t *stack = stackalloc(uint32_t, m_treeHeight + 1);
 	// Push the root node
 	stack[stackLevel++] = 0;
 
@@ -352,7 +353,7 @@ void SingleBVHTree::TraceRay(const vector3d &start, const vector3d &inv_dir, dou
 	PROFILE_SCOPED()
 
 	int32_t stackLevel = 0;
-	uint32_t stack[m_treeHeight + 1];
+	uint32_t *stack = stackalloc(uint32_t, m_treeHeight + 1);
 	stack[stackLevel++] = 0;
 
 	while (stackLevel > 0) {

diff --git a/src/core/macros.h b/src/core/macros.h
@@ -37,3 +37,14 @@
 template <typename T, size_t N>
 char (&COUNTOF_Helper(T (&array)[N]))[N];
 #define COUNTOF(array) (sizeof(COUNTOF_Helper(array)))
+
+// Helper to implement stack-based variable-length arrays in a crossplatform way
+// Avoids a heap allocation for std::vector in "hot" code
+
+#ifdef _MSC_VER
+#include <malloc.h>
+#define stackalloc(T, n) reinterpret_cast<T *>(_alloca(sizeof(T) * n))
+#else
+#include <alloca.h>
+#define stackalloc(T, n) reinterpret_cast<T *>(alloca(sizeof(T) * n))
+#endif