[llvm-dev] Function calls keep increasing the stack usage

[Matthias Braun via llvm-dev]

> On Sep 17, 2018, at 1:30 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Mon, Sep 17, 2018 at 1:27 PM Matthias Braun via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> - Questions like this are better suited for bugs.llvm.org <http://bugs.llvm.org/>
> 
> Not sure - I think it's pretty fine for discussion here, when someone's not sure it's a bug/intended, etc.
>  
> - Please provide the full clang invocation. In this case I suspect your problem will go away if you use `-O2` or similar.
> 
> Right, I think the original post calls this out as specifically being about -O0.
Ah sorry, missed that bit.

It's a bit harder to argue about unoptimize/-O0 code quality. If you can improve the generated code without making the compiler slower (and without increasing complexity) then that may be a change we want to make, but generally we rather want fast/simple code for the -O0 case.
Concrete case here does indeed come out with dead values out of fast-isel. Deciding whether this can be improved inside fast-isel or whether adding a dead code elimination pass will affect compile time would need someone to investigate/benchmark...

- Matthias

> 
> - Dave
>  
> 
> - Matthias
> 
> 
>> On Sep 14, 2018, at 8:16 AM, palpar via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hi everyone,
>> 
>> I found that LLVM generates redundant code when calling functions with constant parameters, with optimizations disabled.
>> 
>> Consider the following C code snippet:
>> 
>> int foo(int x, int y);
>> 
>> void bar()
>> {
>> 	foo(1, 2);
>> 	foo(3, 4);
>> }
>> 
>> Clang/LLVM 6.0 generates the following assembly code:
>> _bar:
>> 	subl	$32, %esp
>> 	movl	$1, %eax
>> 	movl	$2, %ecx
>> 	movl	$1, (%esp)
>> 	movl	$2, 4(%esp)
>> 	movl	%eax, 28(%esp)
>> 	movl	%ecx, 24(%esp)
>> 	calll	_foo
>> 	movl	$3, %ecx
>> 	movl	$4, %edx
>> 	movl	$3, (%esp)
>> 	movl	$4, 4(%esp)
>> 	movl	%eax, 20(%esp)
>> 	movl	%ecx, 16(%esp)
>> 	movl	%edx, 12(%esp)
>> 	calll	_foo
>> 	movl	%eax, 8(%esp)
>> 	addl	$32, %esp
>> 	retl
>> 	
>> Note how the constants are stored in registers but when saving the parameters on the stack for the call the immediate values are used. The registers are still stored on the stack probably because it's the caller's responsibility once they were used (which seems expected).
>> I think the problem comes from the fact that LLVM unconditionally allocates a register for each parameter value regardless if it's used later or not.
>> If the stack space of the program is sufficiently large this is probably not a problem, but otherwise if there is a large number of such calls, despite not recursive, it can lead to stack overflow. Do you think I should create a bug report for this?
>> 
>> (Similarly, the return value of the function could be not saved but the LLVM IR code that Clang generates has the call with assignment so at this point LLVM couldn't possibly know.
>> define void @bar() #0 {
>>   %call = call i32 @foo(i32 1, i32 2)
>>   %call1 = call i32 @foo(i32 3, i32 4)
>>   ret void
>> }
>> )
>> 
>> Thanks,
>> Alpar
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> 
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