[llvm] Revert "[ADT] Always use 32-bit size type for SmallVector with 16-bit elements" (PR #96826)

[Chelsea Cassanova via llvm-commits]

https://github.com/chelcassanova created https://github.com/llvm/llvm-project/pull/96826

Reverts llvm/llvm-project#95536, this is breaking macOS GreenDragon buildbots on arm64 and x86_64 https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/as-lldb-cmake/6522/console, also breaks the Darwin LLVM buildbots: https://lab.llvm.org/buildbot/#/builders/23/builds/398

>From 00dcd9a85ca77ee5e19fa90353b8bab361de983e Mon Sep 17 00:00:00 2001
From: Chelsea Cassanova <chelsea at cassanova.me>
Date: Wed, 26 Jun 2024 15:21:36 -0700
Subject: [PATCH] =?UTF-8?q?Revert=20"[ADT]=20Always=20use=2032-bit=20size?=
 =?UTF-8?q?=20type=20for=20SmallVector=20with=2016-bit=20element=E2=80=A6"?=
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This reverts commit 2582d11f1a8a5783828156d3ced354727f422885.
---
 llvm/include/llvm/ADT/SmallVector.h |  3 ++-
 llvm/lib/Support/SmallVector.cpp    | 12 ++++--------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index db34b16ecf9e7..09676d792dfeb 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -116,7 +116,8 @@ template <class Size_T> class SmallVectorBase {
 
 template <class T>
 using SmallVectorSizeType =
-    std::conditional_t<sizeof(T) == 1, size_t, uint32_t>;
+    std::conditional_t<sizeof(T) < 4 && sizeof(void *) >= 8, uint64_t,
+                       uint32_t>;
 
 /// Figure out the offset of the first element.
 template <class T, typename = void> struct SmallVectorAlignmentAndSize {
diff --git a/llvm/lib/Support/SmallVector.cpp b/llvm/lib/Support/SmallVector.cpp
index e77b747984173..b6ce37842040b 100644
--- a/llvm/lib/Support/SmallVector.cpp
+++ b/llvm/lib/Support/SmallVector.cpp
@@ -37,7 +37,9 @@ struct Struct32B {
 #pragma GCC diagnostic pop
 #endif
 }
-
+static_assert(sizeof(SmallVector<void *, 0>) ==
+                  sizeof(unsigned) * 2 + sizeof(void *),
+              "wasted space in SmallVector size 0");
 static_assert(alignof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
               "wrong alignment for 16-byte aligned T");
 static_assert(alignof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
@@ -46,19 +48,13 @@ static_assert(sizeof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
               "missing padding for 16-byte aligned T");
 static_assert(sizeof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
               "missing padding for 32-byte aligned T");
-
-static_assert(sizeof(SmallVector<void *, 0>) ==
-                  sizeof(unsigned) * 2 + sizeof(void *),
-              "wasted space in SmallVector size 0");
 static_assert(sizeof(SmallVector<void *, 1>) ==
                   sizeof(unsigned) * 2 + sizeof(void *) * 2,
               "wasted space in SmallVector size 1");
+
 static_assert(sizeof(SmallVector<char, 0>) ==
                   sizeof(void *) * 2 + sizeof(void *),
               "1 byte elements have word-sized type for size and capacity");
-static_assert(sizeof(SmallVector<int16_t, 0>) ==
-                  sizeof(unsigned) * 2 + sizeof(void *),
-              "2 byte elements have 32-bit type for size and capacity");
 
 /// Report that MinSize doesn't fit into this vector's size type. Throws
 /// std::length_error or calls report_fatal_error.