[llvm] [Allocator] Keep bump pointer at a minimum alignment (PR #203718)

[Alexis Engelke via llvm-commits]

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@@ -150,30 +161,31 @@ class BumpPtrAllocatorImpl
     // Keep track of how many bytes we've allocated.
     BytesAllocated += Size;
 
-    uintptr_t AlignedPtr = alignAddr(CurPtr, Alignment);
-
     size_t SizeToAllocate = Size;
 #if LLVM_ADDRESS_SANITIZER_BUILD
     // Add trailing bytes as a "red zone" under ASan.
     SizeToAllocate += RedZoneSize;
 #endif
+    SizeToAllocate = alignToPowerOf2(SizeToAllocate, MinAlign);
 
-    uintptr_t AllocEndPtr = AlignedPtr + SizeToAllocate;
-    assert(AllocEndPtr >= uintptr_t(CurPtr) &&
+    // CurPtr is already MinAlign-aligned, so only a stricter request realigns.
+    char *Ptr = CurPtr;
+    if (Alignment.value() > MinAlign)
+      Ptr = reinterpret_cast<char *>(alignAddr(Ptr, Alignment));
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aengelke wrote:

I'm wondering whether we get optimization improvements when using __builtin_align_up with Clang and avoid all pointer-to-int casts. That would avoid exposing the provenance and could help alias analysis -- not sure if there's a practical effect, though.

https://github.com/llvm/llvm-project/pull/203718