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AK: Convince GCC that m_outline_capacity isn't being read

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Previously GCC came to the conclusion that we were reading
m_outline_capacity via ByteBuffer(ByteBuffer const&) -> grow()
-> capacity() even though that could never be the case because
m_size is 0 at that point which means we have an inline buffer
and capacity() would return inline_capacity in that case without
reading m_outline_capacity.

This makes GCC inline parts of the grow() function into the
ByteBuffer copy constructor which seems sufficient for GCC to
realize that m_outline_capacity isn't actually being read.
This commit is contained in:
Gunnar Beutner 2021-05-27 10:52:46 +02:00 committed by Andreas Kling
parent c1a4dfeffb
commit bacb2dea70
Notes: sideshowbarker 2024-07-18 17:17:43 +09:00 
Author: https://github.com/gunnarbeutner
Commit: https://github.com/SerenityOS/serenity/commit/bacb2dea70d
Pull-request: https://github.com/SerenityOS/serenity/pull/7498
1 changed files with 22 additions and 17 deletions
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39
AK/ByteBuffer.h
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@ -146,7 +146,7 @@ public:
m_size = 0;
}
void grow(size_t new_size)
ALWAYS_INLINE void grow(size_t new_size)
{
if (new_size <= m_size)
return;
@ -154,19 +154,7 @@ public:
m_size = new_size;
return;
}
u8* new_buffer;
auto new_capacity = kmalloc_good_size(new_size);
if (!is_inline()) {
new_buffer = (u8*)krealloc(m_outline_buffer, new_capacity);
VERIFY(new_buffer);
} else {
new_buffer = (u8*)kmalloc(new_capacity);
VERIFY(new_buffer);
__builtin_memcpy(new_buffer, data(), m_size);
}
m_outline_buffer = new_buffer;
m_outline_capacity = new_capacity;
m_size = new_size;
grow_slowpath(new_size);
}
void append(void const* data, size_t data_size)
@ -230,12 +218,29 @@ private:
m_size = size;
}
bool is_inline() const { return m_size <= inline_capacity; }
size_t capacity() const { return is_inline() ? inline_capacity : m_outline_capacity; }
NEVER_INLINE void grow_slowpath(size_t new_size)
{
u8* new_buffer;
auto new_capacity = kmalloc_good_size(new_size);
if (!is_inline()) {
new_buffer = (u8*)krealloc(m_outline_buffer, new_capacity);
VERIFY(new_buffer);
} else {
new_buffer = (u8*)kmalloc(new_capacity);
VERIFY(new_buffer);
__builtin_memcpy(new_buffer, data(), m_size);
}
m_outline_buffer = new_buffer;
m_outline_capacity = new_capacity;
m_size = new_size;
}
ALWAYS_INLINE bool is_inline() const { return m_size <= inline_capacity; }
ALWAYS_INLINE size_t capacity() const { return is_inline() ? inline_capacity : m_outline_capacity; }
size_t m_size { 0 };
union {
u8 m_inline_buffer[inline_capacity] {};
u8 m_inline_buffer[inline_capacity];
struct {
u8* m_outline_buffer;
size_t m_outline_capacity;