How to avoid constant propagation into functions?

[Georg-Johann Lay]

On 05.12.2016 17:40, Georg-Johann Lay wrote:
> On 05.12.2016 17:33, Tim Prince wrote:
>>> Hi, I just came across a small test program that propagates a constant
>>> value from a function to a callee, even though the callee has attribute
>>> "noclone".
>>>
>>> From the gcc documentation I would conclude that "noclone" should
>>> prevent such propagations?
>>>
>>> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-g_t_0040code_007bnoclone_007d-function-attribute-3345
>>>
>>>
>>> noclone
>>>     This function attribute prevents a function from being
>>>     considered for cloning -- a mechanism that produces specialized
>>>     copies of functions and which is (currently) performed by
>>>     interprocedural constant propagation.
>>>
>>>
>>> I see this with gcc 5.4 (x86_64) and current trunk.
>>>
>>> Johann
>>
>> Maybe you want -fno-inline-functions ?
>
> The function has __attribute((noinline,noclone)).  This is not sufficient?
>
> Johann

...to be more specific, attached is a C test case.

Function mod_mul is noinline + noclone, and is called with
n=317.  mod_mul itself does not use "317" anywhere.

Yet when compiling with

$ gcc mod.c -Os -save-temps -dp

$ gcc --version
gcc (GCC) 7.0.0 20161202 (experimental) [trunk revision 227650]

and reading asm, there is:

mod_mul:
.LFB1:
	.cfi_startproc
	cmpl	$316, %edx	# 9	*cmpsi_1/1	[length = 6]
	pushq	%rbx	# 84	*pushdi2_rex64/1	[length = 1]
	.cfi_def_cfa_offset 16
	.cfi_offset 3, -16
	movl	%esi, %ebx	# 3	*movsi_internal/1	[length = 2]
	movl	%edx, %esi	# 4	*movsi_internal/1	[length = 2]
	jbe	.L2	# 10	*jcc_1	[length = 2]
	movl	$1, %edx	# 12	*movsi_internal/1	[length = 5]
	movl	$317, %edi	# 14	*movsi_internal/1	[length = 5]
	call	mod_mul	# 15	*call_value	[length = 5]

So the constant 317 made its way into the code of mod_mul (insn 14),
and insn 9 also uses this value.

 From my understanding "noclone" should avoid any such propagations?



Johann

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