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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 14 11:15:53 PDT 2018

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 .cfi_startproc

%bb.0: # %entry

    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register %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
    .cfi_def_cfa %rsp, 8
    retq

Or on 32-bit X86:

bar: # @bar .cfi_startproc

%bb.0: # %entry

    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register %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
    .cfi_def_cfa %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

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