How to avoid constant propagation into functions?

[Georg-Johann Lay]

On 07.12.2016 13:14, Segher Boessenkool wrote:
> On Wed, Dec 07, 2016 at 12:40:34PM +0100, Georg-Johann Lay wrote:
>> ...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?
>
> noclone prevents cloning the function, and it indeed has not been cloned.
>
> Because the function is static, GCC knows all callers, and it determines
> all callers have n=317, so it optimises with that knowledge.
>
> Segher

I understand this.  What I am after is a means to get a generated
code that matches the source, no matter what context.

Imagine you called that function from assembler, then the code
would crash.  And yes, when I am adding some other usage of
mod_mul or let escape its address, then the propagation is not
performed.

For example, this might have impact on writing test for GCC:

When I am writing a test with noinline + noclone then my
expectation is that no such propagation happens, because
otherwise a test might turn trivial...

Johann



implementation