(In reply to Walter Bright from comment #1)
> Please provide a reproducible example.
I already spend almost a day on this and couldn't reduce it to less than compiling Higgs and running the tests. Will try again, but it's definitely cause by the mentioned commit.
Comment #3 by code — 2015-02-02T01:24:27Z
After spending yet another day on this :(, I finally found it.
It's a bug in Higgs that was triggered by the change to the struct literal code.
https://github.com/higgsjs/Higgs/pull/177
I'm wondering why this worked before.
Was the struct previously fully initialized before calling the ctor, or did this just work by accidentally?
Anyhow, I filed bug 14107 because this should never have compiled.
Comment #4 by code — 2015-02-05T23:34:24Z
struct X86Imm
{
ulong imm;
}
struct X86Opnd
{
union
{
X86Reg reg;
X86Imm imm;
}
ubyte tag;
this(X86Reg r) { reg = r; }
}
struct X86Reg
{
/// Register type
ubyte type;
/// Register index number
ubyte regNo;
/// Size in bits
ushort size;
}
X86Opnd opnd(X86Reg reg)
{
return X86Opnd(reg);
}
Here is the asm that for the opnd function with 2.066.1
_D4bug24opndFS4bug26X86RegZS4bug27X86Opnd PROC
push rbp ; 0000 _ 55
mov rbp, rsp ; 0001 _ 48: 8B. EC
sub rsp, 32 ; 0004 _ 48: 83. EC, 20
lea rax, [rbp-20H] ; 0008 _ 48: 8D. 45, E0
xor rcx, rcx ; 000C _ 48: 31. C9
mov qword ptr [rax], rcx ; 000F _ 48: 89. 08
mov qword ptr [rax+8H], rcx ; 0012 _ 48: 89. 48, 08
mov rsi, rdi ; 0016 _ 48: 89. FE
mov rdi, rax ; 0019 _ 48: 89. C7
call _D4bug27X86Opnd6__ctorMFNcS4bug26X86RegZS4bug27X86Opnd; 001C _ E8, 00000000(rel)
mov rdx, qword ptr [rax+8H] ; 0021 _ 48: 8B. 50, 08
mov rax, qword ptr [rax] ; 0025 _ 48: 8B. 00
mov rsp, rbp ; 0028 _ 48: 8B. E5
pop rbp ; 002B _ 5D
ret ; 002C _ C3
_D4bug24opndFS4bug26X86RegZS4bug27X86Opnd ENDP
That's the same function now.
_D4bug24opndFS4bug26X86RegZS4bug27X86Opnd PROC
push rbp ; 0000 _ 55
mov rbp, rsp ; 0001 _ 48: 8B. EC
sub rsp, 32 ; 0004 _ 48: 83. EC, 20
xor eax, eax ; 0008 _ 31. C0
mov dword ptr [rbp-18H], eax ; 000A _ 89. 45, E8
mov dword ptr [rbp-14H], eax ; 000D _ 89. 45, EC
xor ecx, ecx ; 0010 _ 31. C9
mov byte ptr [rbp-10H], cl ; 0012 _ 88. 4D, F0
mov byte ptr [rbp-0FH], cl ; 0015 _ 88. 4D, F1
mov word ptr [rbp-0EH], ax ; 0018 _ 66: 89. 45, F2
mov edx, dword ptr [rbp-10H] ; 001C _ 8B. 55, F0
mov dword ptr [rbp-20H], edx ; 001F _ 89. 55, E0
mov byte ptr [rbp-18H], cl ; 0022 _ 88. 4D, E8
mov rsi, rdi ; 0025 _ 48: 89. FE
lea rdi, [rbp-20H] ; 0028 _ 48: 8D. 7D, E0
call _D4bug27X86Opnd6__ctorMFNcS4bug26X86RegZS4bug27X86Opnd; 002C _ E8, 00000000(rel)
mov rdx, qword ptr [rax+8H] ; 0031 _ 48: 8B. 50, 08
mov rax, qword ptr [rax] ; 0035 _ 48: 8B. 00
mov rsp, rbp ; 0038 _ 48: 8B. E5
pop rbp ; 003B _ 5D
ret ; 003C _ C3
_D4bug24opndFS4bug26X86RegZS4bug27X86Opnd ENDP
Just focusing on the X86Opnd init code
lea rax, [rbp-20H]
xor rcx, rcx
mov qword ptr [rax], rcx
mov qword ptr [rax+8H], rcx
In the new code the 4 bytes at [rbp-1CH] are never initialized.
xor eax, eax
mov dword ptr [rbp-18H], eax
mov dword ptr [rbp-14H], eax
xor ecx, ecx
mov byte ptr [rbp-10H], cl
mov byte ptr [rbp-0FH], cl
mov word ptr [rbp-0EH], ax
mov edx, dword ptr [rbp-10H]
mov dword ptr [rbp-20H], edx
mov byte ptr [rbp-18H], cl
mov rsi, rdi
lea rdi, [rbp-20H]
Without a constructor it's the same for both dmd versions and also only initializes the first union field of X86Opnd.
xor eax, eax
mov dword ptr [rbp-18H], eax
mov dword ptr [rbp-14H], eax
mov dword ptr [rbp-20H], edi
So this isn't really a regression and it was a bug in Higgs to rely on memory compare.
I filed https://issues.dlang.org/show_bug.cgi?id=14107 to disallow default comparison for structs with unions.
The amount of code generated for the constructor is still worrisome.
Especially the part that initializes X86Reg, because it initializes every field individually, it constructs a temporary on the stack only to end up loading zero into edx.
xor ecx, ecx
mov byte ptr [rbp-10H], cl
mov byte ptr [rbp-0FH], cl
mov word ptr [rbp-0EH], ax
mov edx, dword ptr [rbp-10H]
So we should still consider to revert that part of the change.