Re: detecting uninitialized values in debug builds

Thanks.  Obviously, I should have tried that before posting my question, although I would not have known how to obtain the answer without seeing your answer below.

Not being familiar with the x86 asm language, I modified the foo.cpp program (explicitly assign y=0) and then g++ again, so I could compare the two asm lang outputs.  After that, it was fairly clear that the compiler is not initializing y.

Ben

-----Original Message-----
From: John Reiser <[email protected]> 
Sent: Monday, March 2, 2020 7:20 PM
To: [email protected]
Subject: Re: [Valgrind-users] detecting uninitialized values in debug builds

On 3/2/20 22:02 UTC, Ben White wrote:
> I've also been told that the g++ compiler will initialize all stack 
> objects to zero when compiling for debug (the -g option).

Obviously you didn't try it.  g++ 9.2.1 does not do that.

$ cat foo.cpp
int g(int x, int y);

int f(int x)
{
	int y;
         return	g(x, y);  // uninit use of y
}

$ g++ -g -S foo.cpp
$ cat foo.s
	.file	"foo.cpp"
	.text
.Ltext0:
	.globl	_Z1fi
	.type	_Z1fi, @function
_Z1fi:
.LFB0:
	.file 1 "foo.cpp"
	.loc 1 4 1
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	subq	$32, %rsp
	movl	%edi, -20(%rbp)
	.loc 1 6 17
	movl	-4(%rbp), %edx   // -4(%rbp) is never initialized
	movl	-20(%rbp), %eax
	movl	%edx, %esi   // 2nd parameter to g
	movl	%eax, %edi
	call	_Z1gii
	.loc 1 7 1
	leave
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	_Z1fi, .-_Z1fi
   [[snip]]
	.section	.debug_str,"MS",@progbits,1
.LASF1:
	.string	"foo.cpp"
.LASF0:
	.string	"GNU C++14 9.2.1 20190827 (Red Hat 9.2.1-1) -mtune=generic -march=x86-64 -g"
.LASF3:
	.string	"_Z1fi"
.LASF2:
	.string	"/home/user"
	.ident	"GCC: (GNU) 9.2.1 20190827 (Red Hat 9.2.1-1)"
	.section	.note.GNU-stack,"",@progbits
$


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