[llvm-dev] understanding llvm's codegen for function forwarding

[Nicolai Hähnle via llvm-dev]

I'd assume -fomit-frame-pointer would make a difference.

Cheers,
Nicolai

On 23.11.18 20:49, Andrew Kelley via llvm-dev wrote:
> When compiling this LLVM IR with -O0 (no optimizations)
> 
> define internal fastcc void @bar2(%Bar* nonnull sret) unnamed_addr #2 !dbg !74 {
> Entry:
>    call fastcc void @bar(%Bar* sret %0), !dbg !79
>    ret void, !dbg !81
> }
> 
> why does this generate this?
> 
> 0000000000000090 <bar2>:
>    90:    55                       push   %rbp
>    91:    48 89 e5                 mov    %rsp,%rbp
>    94:    48 83 ec 10              sub    $0x10,%rsp
>    98:    48 89 f8                 mov    %rdi,%rax
>    9b:    48 89 45 f8              mov    %rax,-0x8(%rbp)
>    9f:    e8 0c 00 00 00           callq  b0 <bar>
>    a4:    48 8b 45 f8              mov    -0x8(%rbp),%rax
>    a8:    48 83 c4 10              add    $0x10,%rsp
>    ac:    5d                       pop    %rbp
>    ad:    c3                       retq
>    ae:    66 90                    xchg   %ax,%ax
> 
> 
> instead of something like this?
> 
> 0000000000000090 <bar2>:
>    9f:    e8 0c 00 00 00           callq  b0 <bar>
>    ad:    c3                       retq
> 
> when I add `musttail` to the IR it gives me this assembly:
> 
> 00000000000000a0 <bar2>:
>    a0:    55                       push   %rbp
>    a1:    48 89 e5                 mov    %rsp,%rbp
>    a4:    48 83 ec 10              sub    $0x10,%rsp
>    a8:    48 89 f8                 mov    %rdi,%rax
>    ab:    48 89 45 f8              mov    %rax,-0x8(%rbp)
>    af:    48 83 c4 10              add    $0x10,%rsp
>    b3:    5d                       pop    %rbp
>    b4:    e9 07 00 00 00           jmpq   c0 <bar>
>    b9:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)
> 
> which does not have a call instruction but it has prologue that I
> would not expect.
> 
> What's going on here? Is this something that can not really be
> improved without optimization passes?
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