Hi all,
I'm trying to generate IR with debug option (-g) for the following code
snippet.
int func(int i) {
return i+1;
}
The corresponding IR for the function definition is:
define i32 @func(i32 %i) #0 !dbg !7 {
entry:
%i.addr = alloca i32, align 4
store i32 %i, i32* %i.addr, align 4
call void @llvm.dbg.declare(metadata i32* %i.addr, metadata !11, metadata
!12), !dbg !13
%0 = load i32, i32* %i.addr, align 4, !dbg !14
%add = add nsw i32 %0, 1, !dbg !15
ret i32 %add, !dbg !16
}
I'm confused about why the "*call void @llvm.dbg.declare(metadata i32*
%i.addr, metadata !11, metadata !12), !dbg !13*" appears after an usage of
*%i.addr* (*store i32 %i, i32* %i.addr, align 4*)*. *Should the use of*
%i.addr* happen after llvm.dbg.declare or does it have any specific reason
for this? Am I missing something here?
Thank you very much,
Akash Banerjee
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190330/18853168/attachment.html>
Hi Akash, On Sat, 30 Mar 2019 at 13:52, Akash Banerjee via llvm-dev <llvm-dev at lists.llvm.org> wrote:> I'm confused about why the "call void @llvm.dbg.declare(metadata i32* %i.addr, metadata !11, metadata !12), !dbg !13" appears after an usage of %i.addr (store i32 %i, i32* %i.addr, align 4).Putting the dbg.declare (tagged with the first line of the function) after that initial store ensures that a debugger will never see an invalid value for formal parameters like that. Whenever a debugger stops on the line, "i" will have been initialized from the incoming register (or stack slot), which seems like a good thing to me. On the other hand Clang doesn't seem so picky for normal local variables, which only get their stable value on the next line after they've been declared. So it's quite likely (IMO) that it's just a quirk of how Clang goes about generating IR and the order it does things in (generate allocas -> call target-specific code to marshal args but don't make it worry about debugging info -> officially declare any args). You might get a more definitive answer to "why" if you asked this in the cfe-dev mailing list, especially if you catch the attention of whoever wrote the code. Here we're more able to answer questions on the implications of that order for debugging. Cheers. Tim.