[libc-commits] [libc] [libc] Fix chunk calculation in GPU allocator (PR #221583)

[Joseph Huber via libc-commits]

https://github.com/jhuber6 created https://github.com/llvm/llvm-project/pull/221583

Summary:
This would pesismistically round up 48 to 64 and the previous s0 case
was unused.


>From 5e5f32bdada363afc75a822d6e161f80762b3ade Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Sun, 6 Sep 2026 09:59:09 -0500
Subject: [PATCH] [libc] Fix chunk calculation in GPU allocator

Summary:
This would pesismistically round up 48 to 64 and the previous s0 case
was unused.
---
 libc/src/__support/GPU/allocator.cpp | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/libc/src/__support/GPU/allocator.cpp b/libc/src/__support/GPU/allocator.cpp
index c88a70708b964..75b9ea73e609c 100644
--- a/libc/src/__support/GPU/allocator.cpp
+++ b/libc/src/__support/GPU/allocator.cpp
@@ -94,24 +94,21 @@ static inline uint32_t xorshift32(uint32_t &state) {
 }
 
 // Rounds the input value to the closest permitted chunk size. Here we accept
-// the sum of the closest three powers of two. For a 2MiB slab size this is 48
+// the sum of the closest three powers of two. For a 2MiB slab size this is 46
 // different chunk sizes. This gives us average internal fragmentation of 87.5%.
 static inline constexpr uint32_t get_chunk_size(uint32_t x) {
   uint32_t y = x < MIN_SIZE ? MIN_SIZE : x;
   uint32_t pow2 = BITS_IN_WORD - cpp::countl_zero(y - 1);
 
-  uint32_t s0 = 0b0100 << (pow2 - 3);
-  uint32_t s1 = 0b0110 << (pow2 - 3);
-  uint32_t s2 = 0b0111 << (pow2 - 3);
-  uint32_t s3 = 0b1000 << (pow2 - 3);
+  uint32_t s0 = 0b0110 << (pow2 - 3);
+  uint32_t s1 = 0b0111 << (pow2 - 3);
+  uint32_t s2 = 0b1000 << (pow2 - 3);
 
-  if (s0 > y)
+  if (s0 >= y)
     return (s0 + MIN_ALIGNMENT) & ~MIN_ALIGNMENT;
-  if (s1 > y)
+  if (s1 >= y)
     return (s1 + MIN_ALIGNMENT) & ~MIN_ALIGNMENT;
-  if (s2 > y)
-    return (s2 + MIN_ALIGNMENT) & ~MIN_ALIGNMENT;
-  return (s3 + MIN_ALIGNMENT) & ~MIN_ALIGNMENT;
+  return (s2 + MIN_ALIGNMENT) & ~MIN_ALIGNMENT;
 }
 
 // Converts a chunk size into an index suitable for a statically sized array.