[llvm] Reduce binary size of the Microsoft demangler (PR #89454)

[via llvm-commits]

https://github.com/apache-hb created https://github.com/llvm/llvm-project/pull/89454

This pulls the body of `llvm::ms_demangle::ArenaAllocator::allocArray` and `llvm::ms_demangle::ArenaAllocator::alloc` into a common `allocAlignedBuffer` function. In my testing this reduces the overall binary size of `LLVMDemangle.lib` by 6kb and has a negligible impact on performance.

The `addNode(std::max(AllocUnit, Size));` being pulled into the common function retains the same behaviour as before as `Size` will always be `< AllocUnit` when allocating a single object.

>From 8f3933c2b74edf999a97450c015bd9712c15e96e Mon Sep 17 00:00:00 2001
From: Elliot <35050275+apache-hb at users.noreply.github.com>
Date: Mon, 15 Apr 2024 03:47:04 -0400
Subject: [PATCH 1/2] Reduce binary size of the microsoft demangler

---
 .../include/llvm/Demangle/MicrosoftDemangle.h | 55 ++++++++-----------
 1 file changed, 22 insertions(+), 33 deletions(-)

diff --git a/llvm/include/llvm/Demangle/MicrosoftDemangle.h b/llvm/include/llvm/Demangle/MicrosoftDemangle.h
index 1529b803debe5a..336b8b9968088c 100644
--- a/llvm/include/llvm/Demangle/MicrosoftDemangle.h
+++ b/llvm/include/llvm/Demangle/MicrosoftDemangle.h
@@ -41,6 +41,24 @@ class ArenaAllocator {
     NewHead->Used = 0;
   }
 
+  uint8_t *allocAlignedBuffer(size_t Size, size_t Align) {
+    assert(Head && Head->Buf);
+
+    size_t P = (size_t)Head->Buf + Head->Used;
+    uintptr_t AlignedP =
+        (((size_t)P + Align - 1) & ~(size_t)(Align - 1));
+    uint8_t *PP = (uint8_t *)AlignedP;
+    size_t Adjustment = AlignedP - P;
+
+    Head->Used += Size + Adjustment;
+    if (Head->Used <= Head->Capacity)
+      return PP;
+
+    addNode(std::max(AllocUnit, Size));
+    Head->Used = Size;
+    return Head->Buf;
+  }
+
 public:
   ArenaAllocator() { addNode(AllocUnit); }
 
@@ -69,42 +87,13 @@ class ArenaAllocator {
   }
 
   template <typename T, typename... Args> T *allocArray(size_t Count) {
-    size_t Size = Count * sizeof(T);
-    assert(Head && Head->Buf);
-
-    size_t P = (size_t)Head->Buf + Head->Used;
-    uintptr_t AlignedP =
-        (((size_t)P + alignof(T) - 1) & ~(size_t)(alignof(T) - 1));
-    uint8_t *PP = (uint8_t *)AlignedP;
-    size_t Adjustment = AlignedP - P;
-
-    Head->Used += Size + Adjustment;
-    if (Head->Used <= Head->Capacity)
-      return new (PP) T[Count]();
-
-    addNode(std::max(AllocUnit, Size));
-    Head->Used = Size;
-    return new (Head->Buf) T[Count]();
+    uint8_t *Data = allocAlignedBuffer(Count * sizeof(T), alignof(T));
+    return new (Data) T[Count]();
   }
 
   template <typename T, typename... Args> T *alloc(Args &&... ConstructorArgs) {
-    constexpr size_t Size = sizeof(T);
-    assert(Head && Head->Buf);
-
-    size_t P = (size_t)Head->Buf + Head->Used;
-    uintptr_t AlignedP =
-        (((size_t)P + alignof(T) - 1) & ~(size_t)(alignof(T) - 1));
-    uint8_t *PP = (uint8_t *)AlignedP;
-    size_t Adjustment = AlignedP - P;
-
-    Head->Used += Size + Adjustment;
-    if (Head->Used <= Head->Capacity)
-      return new (PP) T(std::forward<Args>(ConstructorArgs)...);
-
-    static_assert(Size < AllocUnit);
-    addNode(AllocUnit);
-    Head->Used = Size;
-    return new (Head->Buf) T(std::forward<Args>(ConstructorArgs)...);
+    uint8_t *Data = allocAlignedBuffer(sizeof(T), alignof(T));
+    return new (Data) T(std::forward<Args>(ConstructorArgs)...);
   }
 
 private:

>From f970420852ff1370909871961bb9f6ea497f07b9 Mon Sep 17 00:00:00 2001
From: Elliot <35050275+apache-hb at users.noreply.github.com>
Date: Fri, 19 Apr 2024 16:56:08 -0400
Subject: [PATCH 2/2] Update MicrosoftDemangle.h

---
 llvm/include/llvm/Demangle/MicrosoftDemangle.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/include/llvm/Demangle/MicrosoftDemangle.h b/llvm/include/llvm/Demangle/MicrosoftDemangle.h
index 336b8b9968088c..6ce03ebb0dea3f 100644
--- a/llvm/include/llvm/Demangle/MicrosoftDemangle.h
+++ b/llvm/include/llvm/Demangle/MicrosoftDemangle.h
@@ -92,6 +92,7 @@ class ArenaAllocator {
   }
 
   template <typename T, typename... Args> T *alloc(Args &&... ConstructorArgs) {
+    static_assert(sizeof(T) < AllocUnit);
     uint8_t *Data = allocAlignedBuffer(sizeof(T), alignof(T));
     return new (Data) T(std::forward<Args>(ConstructorArgs)...);
   }