[Issue 15573] -O -inline causes wrong code with idiv instruction

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15573

--- Comment #30 from yebblies <yebblies at gmail.com> ---
(In reply to hsteoh from comment #29)
> 
> In the bad (optimized) version at the top, however, it's comparing the local
> variable `m` (which, from an earlier part of the dump, can be seen to be
> stored in -0x8(%rbp)) against %dh, which doesn't seem to make sense. Why
> does the compiler emit a comparison agains %dh instead of 0x0?  Is it
> assuming that %dh is zero? Why would it make such as assumption?
> 
> After this point, the value of div0 is wrong, and the code proceeds down a
> different path than it should.

Yeah, the compiler is incorrectly assuming that dh contains zero.  In cdnot in
cod2.c, there is the check

if (reghasvalue((sz == 1) ? BYTEREGS : ALLREGS,0,&reg))

which uses the cmp mem, reg form if it finds a register that contains zero. 
I'm not sure why it returns true and sets reg to 6 though.

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