[Issue 20466] New: Optimizer clobbering msw register when testing it for 0

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20466

          Issue ID: 20466
           Summary: Optimizer clobbering msw register when testing it for
                    0
           Product: D
           Version: D2
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: critical
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Compile the following code with -O -m32 -fPIC and then
disassemble it:
-------------------------
enum STC : long {
    auto_ = (1L << 8),
    trusted = (1L << 34),
}

alias StorageClass = ulong;

extern (C++) final class Parameter {
    StorageClass storageClass;
    void* type;
}

extern (C++) final class IfStatement {
    Parameter prm;
}

extern (C++) final class Visitor {
    void visit(IfStatement s) {
        if (Parameter p = s.prm) {
            StorageClass stc = p.storageClass;
            if (!p.type && !stc)
                stc = STC.auto_;
            assert(!(stc & STC.trusted));
        }
    }
}
----------------
_ZN7Visitor5visitEP11IfStatement:
                push    EBP
                mov     EBP,ESP
                sub     ESP,018h
                mov     -0Ch[EBP],EBX
                mov     -8[EBP],ESI
                call      L11
L11:            pop     EAX
                add     EAX,offset FLAT:_GLOBAL_OFFSET_TABLE_ at GOTPC[2]
                mov     -4[EBP],EAX
                mov     ECX,0Ch[EBP]
                mov     EDX,4[ECX]
                test    EDX,EDX
                je      L4B
                cmp     dword ptr 0Ch[EDX],0
                mov     ECX,8[EDX]
                mov     EBX,4[EDX]
                jne     L3B
                or      ECX,EBX    <=== clobbers ECX
                jne     L3B
                mov     EBX,0100h
                xor     ECX,ECX
L3B:            mov     EAX,EBX
                mov     EDX,ECX    <=== using clobbered value here
                xor     EAX,EAX
                and     EDX,4
                or      EDX,EAX
                jne     L55
L4B:            mov     EBX,-0Ch[EBP]
                mov     ESI,-8[EBP]
                mov     ESP,EBP
                pop     EBP
                ret
L55:            mov     dword ptr -014h[EBP],01Dh
                mov     EBX,-4[EBP]
                lea     ESI,_TMP0 at GOTOFF[061h][EBX]
                mov     -018h[EBP],ESI
                mov     EBX,-4[EBP]
                call      _d_assertp at PLT32
                mov     EBX,-0Ch[EBP]
                mov     ESI,-8[EBP]
                mov     ESP,EBP
                pop     EBP
                ret
----------------
See where the register is clobbered.

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