[llvm] Simplify hot-path size computations in BumpPtrAllocator. (PR #101312)

[Owen Anderson via llvm-commits]

https://github.com/resistor created https://github.com/llvm/llvm-project/pull/101312

~0.1% instruction count improvements

https://llvm-compile-time-tracker.com/compare.php?from=07d2709a17860a202d91781769a88837e4fb5f2a&to=d5cc47831ecd9f0a2b164b16da67f74b94e9aafc&stat=instructions:u

>From d5cc47831ecd9f0a2b164b16da67f74b94e9aafc Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor at mac.com>
Date: Tue, 30 Jul 2024 21:50:15 -1000
Subject: [PATCH] Simplify hot-path size computations in BumpPtrAllocator.

---
 llvm/include/llvm/Support/Allocator.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/Support/Allocator.h b/llvm/include/llvm/Support/Allocator.h
index e05f0ec0e8704..d9a4f6599e6bb 100644
--- a/llvm/include/llvm/Support/Allocator.h
+++ b/llvm/include/llvm/Support/Allocator.h
@@ -149,8 +149,7 @@ class BumpPtrAllocatorImpl
     // Keep track of how many bytes we've allocated.
     BytesAllocated += Size;
 
-    size_t Adjustment = offsetToAlignedAddr(CurPtr, Alignment);
-    assert(Adjustment + Size >= Size && "Adjustment + Size must not overflow");
+    char* AlignedPtr = reinterpret_cast<char*>(alignAddr(CurPtr, Alignment));
 
     size_t SizeToAllocate = Size;
 #if LLVM_ADDRESS_SANITIZER_BUILD
@@ -158,12 +157,13 @@ class BumpPtrAllocatorImpl
     SizeToAllocate += RedZoneSize;
 #endif
 
+    char* AllocEndPtr = AlignedPtr + SizeToAllocate;
+
     // Check if we have enough space.
-    if (LLVM_LIKELY(Adjustment + SizeToAllocate <= size_t(End - CurPtr)
+    if (LLVM_LIKELY(AllocEndPtr <= End
                     // We can't return nullptr even for a zero-sized allocation!
                     && CurPtr != nullptr)) {
-      char *AlignedPtr = CurPtr + Adjustment;
-      CurPtr = AlignedPtr + SizeToAllocate;
+      CurPtr = AllocEndPtr;
       // Update the allocation point of this memory block in MemorySanitizer.
       // Without this, MemorySanitizer messages for values originated from here
       // will point to the allocation of the entire slab.