[llvm-dev] Function calls keep increasing the stack usage (original) (raw)

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 14 12:31:53 PDT 2018

I suspect it was fixed by my local value sinking change, which delete unused local value materializations like these.

On Fri, Sep 14, 2018 at 12:20 PM palpar via llvm-dev < llvm-dev at lists.llvm.org> wrote:

Thanks for checking, I suppose it may have been fixed then, I don't have the latest version to try it now. Curious what could have fixed it, because X86FastISel::fastLowerCall() still has the calls to getRegForValue() (or maybe that's not the problem).

On Fri, Sep 14, 2018 at 10:02 PM David Blaikie <dblaikie at gmail.com> wrote:

Still not seeing it on ToT, so maybe it's been fixed?

$ clang-tot -cc1 -S -triple i386-pc-win32 stack.c ... bar: subl $16, %esp movl $1, (%esp) movl $2, 4(%esp) calll foo movl $3, (%esp) movl $4, 4(%esp) movl %eax, 12(%esp) calll foo movl %eax, 8(%esp) addl $16, %esp retl $ clang-tot --version clang version 8.0.0 (trunk 342200) (llvm/trunk 342202) Target: x8664-unknown-linux-gnu Thread model: posix

On Fri, Sep 14, 2018 at 11:57 AM palpar <palparni at gmail.com> wrote: Sorry I missed that important detail. The relevant part of the command line is: -cc1 -S -triple i386-pc-win32 I don't expect it matters if it's for Windows or Linux in this case. On Fri, Sep 14, 2018 at 9:16 PM David Blaikie <dblaikie at gmail.com> wrote: Can't say I've observed that behavior (though I'm just building from top-of-tree rather than 6.0, compiling for x86-64 on linux), perhaps you could provide more detail (what target are you compiling for - possibly provide the -cc1 command line, etc). bar: # @bar .cfistartproc # %bb.0: # %entry pushq %rbp .cfidefcfaoffset 16 .cfioffset %rbp, -16 movq %rsp, %rbp .cfidefcfaregister %rbp subq $16, %rsp movl $1, %edi movl $2, %esi callq foo movl $3, %edi movl $4, %esi movl %eax, -4(%rbp) # 4-byte Spill callq foo movl %eax, -8(%rbp) # 4-byte Spill addq $16, %rsp popq %rbp .cfidefcfa %rsp, 8 retq

Or on 32-bit X86: bar: # @bar .cfistartproc # %bb.0: # %entry pushq %rbp .cfidefcfaoffset 16 .cfioffset %rbp, -16 movq %rsp, %rbp .cfidefcfaregister %rbp subq $16, %rsp movl $1, %edi movl $2, %esi callq foo movl $3, %edi movl $4, %esi movl %eax, -4(%rbp) # 4-byte Spill callq foo movl %eax, -8(%rbp) # 4-byte Spill addq $16, %rsp popq %rbp .cfidefcfa %rsp, 8 retq On Fri, Sep 14, 2018 at 8:16 AM palpar via llvm-dev <_ _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

LLVM Developers mailing list llvm-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

LLVM Developers mailing list llvm-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180914/b9a7a20c/attachment.html>

More information about the llvm-dev mailing list