CBE miscompiles common C++ Code (bigfib, tramp3d, etc)

[lattner]

Bugzilla Link	1658
Resolution	WONTFIX
Resolved on	Mar 26, 2012 05:57
Version	1.0
OS	All
CC	@asl,@d0k,@edwintorok,@sunfishcode,@nlewycky

Extended Description

Consider this code:

struct ctor {
void *foo;
ctor(const ctor &X);
ctor() {}
};

struct test {
unsigned char *field0;
};

test foo();
ctor bar();

void testfunc() {
test xx;
ctor yy;

xx = foo();
yy = bar();
}

In the GCC abi, single element structs are returned as registers, larger structs and any struct with a copy ctor are returned by passing a hidden "sret" pointer to the struct. llvm-gcc compiles this to:

    %tmp2 = call i8* @&#8203;_Z3foov( )            ; <i8*> [#uses=0]
    call void @&#8203;_Z3barv( %struct.ctor* %tmp1 sret  )

which is correct. The problem is that the CBE doesn't know about this. When run through the CBE, we get:

struct l_struct_2E_ctor {
unsigned char field0;
};
...
struct l_struct_2E_ctor llvm_cbe_tmp1; / Address-exposed local */
*((&llvm_cbe_tmp1)) = _Z3barv();
...

Suddenly the the C compiler compiling the CBE output will return the struct in a register, instead of passing it by dummy argument. This causes miscompilation of Misc-C++/bigfib and probably a lot of other stuff because this affects std::string.

-Chris

[asl]

Hrm :)

[lattner]

ugly huh?

[lattner]

*** Bug llvm/llvm-bugzilla-archive#2125 has been marked as a duplicate of this bug. ***

[nlewycky]

Does sret fix this? Or multiple value ret?

[lattner]

Nope.

[lattner]

*** Bug llvm/llvm-bugzilla-archive#3504 has been marked as a duplicate of this bug. ***

[d0k]

The C backend was removed from svn trunk and is no longer maintained. Closing the bug.

[lattner]

mentioned in issue llvm/llvm-bugzilla-archive#2125

[lattner]

mentioned in issue llvm/llvm-bugzilla-archive#3504