https://bugs.llvm.org/show_bug.cgi?id=45052
Bug ID: 45052
Summary: Missed optimization: bit-field struct with single byte
enum class type.
Product: libraries
Version: 9.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: wnukow...@google.com
CC: llvm-bugs@lists.llvm.org
For a code
enum class enum_type8 : char {
};
struct random_struct {
enum_type8 field1: 8;
char field2: 8;
};
bool Compare(const random_struct& lhs, const random_struct& rhs) {
return lhs.field1 == rhs.field1 && lhs.field2 == rhs.field2;
};
llvm 9 (with -O2) emits:
movzx eax, word ptr [rdi]
movzx ecx, word ptr [rsi]
cmp al, cl
sete dl
xor ecx, eax
movzx eax, cx
cmp eax, 256
setb al
and al, dl
ret
But it could easily emit (as shown in compiler explorer link):
movzx eax, word ptr [rdi]
cmp ax, word ptr [rsi]
sete al
ret
See https://godbolt.org/z/7CMja-
In other words, optimizations are not able to handle the case of a struct with
bit-fields that contains single byte scoped enum. Making the enum type
'inherit' from short suddenly makes the optimizer happy, even when it does not
affect the layout of the struct (in this case { i16 }).
This is just a 'MVP' of a missed optimization. There are few other cases when
codegen behaves differently just because single byte scoped enum is present.
Another example when scoped enum size affects struct layout:
enum class test_enum8 : char {
};
enum class test_enum16 : short {
};
struct struct1 {
bool field : 1;
test_enum8 field2 : 8;
char field3 : 7;
};
struct struct2 {
bool field : 1;
test_enum16 field2 : 8;
char field3 : 7;
};
Results in:
%struct.struct1 = type <{ i8, i16 }>
%struct.struct2 = type { i16 }
But this could easily be packed as { i16 } in both cases. According to C++
standard, bit-field packing is implementation specific, so technically,
generated code is correct.
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