[ADT] Always use 32-bit size type for SmallVector with 16-bit elements (#95536)

`SmallVector` has a special case to allow vector of char to exceed 4 GB
in
size on 64-bit hosts. Apply this special case only for 8-bit element
types, instead of all element types < 32 bits.

This makes `SmallVector<MCPhysReg>` more compact because `MCPhysReg` is
`uint16_t`.

---------

Co-authored-by: Nikita Popov <[email protected]>

Author: 2 people

llvm/include/llvm/ADT/SmallVector.h

@@ -116,8 +116,7 @@ template <class Size_T> class SmallVectorBase {
 
 template <class T>
 using SmallVectorSizeType =
-    std::conditional_t<sizeof(T) < 4 && sizeof(void *) >= 8, uint64_t,
-                       uint32_t>;
+    std::conditional_t<sizeof(T) == 1, size_t, uint32_t>;
 
 /// Figure out the offset of the first element.
 template <class T, typename = void> struct SmallVectorAlignmentAndSize {

llvm/lib/Support/SmallVector.cpp

@@ -37,9 +37,7 @@ 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),
@@ -48,13 +46,19 @@ 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.