[llvm-dev] llvm return value propagation & asm

[Reid Kleckner via llvm-dev]

We need to disable most forms of IPO on naked functions. We already do this
in DeadArgElimination.cpp. We need to do it in more places.

On Fri, Jan 27, 2017 at 5:15 AM, Carlo Kok via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi, I'm trying to have a pure asm function (non inlined) that returns it's
> own value to the caller.
>
> ; Function Attrs: naked noinline optnone
> define i32 @callcatch(i32, i32) #3 !dbg !10103 {
> BasicBlock8472:
>   call void asm "\0D\0Apushl %ebp\0D\0Amovl 8(%esp),%eax\0D\0Amovl
> 12(%esp), %ebp\0D\0Acalll *%eax\0D\0Apopl %ebp\0D\0Aretl\0D\0A", ""(), !dbg
> !10104, !srcloc !10106 // this returns in eax
>   ret i32 0, !dbg !10104
> }
>
> ; Function Attrs: naked noinline optnone
> define void @jumptocont(i32, i32, i32) #3 !dbg !10107 {
> BasicBlock8473:
>   call void asm "\0D\0A    movl 12(%esp), %ebp\0D\0A    movl 4(%esp),
> %eax\0D\0A    movl 8(%esp), %esp\0D\0A    jmpl *%eax\0D\0A", ""(), !dbg
> !10108, !srcloc !10110
>   ret void, !dbg !10108
> }
>
> The calling code is like;
>   %530 = call i32 @callcatch(i32 %528, i32 %529) #3, !dbg !7648
>   store i32 %530, i32* %20, !dbg !7648
>   %531 = load i32, i32* %20, !dbg !7630
> ...
>   call void @jumptocont(i32 %531, i32 %532, i32 %533) #3, !dbg
>
> However the return constant propagation code from
>
> http://llvm.org/docs/doxygen/html/IPConstantPropagation_8cpp_source.html
>
> finds the ret i32 0 and ends up calling jumptocont with i32 0 as a first
> parameter, which wasn't what I wanted. Ending up like:
>
>  %189 = tail call i32 @callcatchn(i32 %188, i32 %12) #10, !dbg !7019
>   tail call void @jumptocont(i32 0, i32 %14, i32 %12) #10, !dbg !7022
>
> I tried unreachable in the callcatch which would make the caller code turn
> into unreachable too. Is there a workaround for this, or a better way to do
> this?
>
> --
> Carlo Kok
> RemObjects Software
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