[llvm-dev] llvm return value propagation & asm

Carlo Kok via llvm-dev
Fri, 27 Jan 2017 13:15:36 -0000

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