<div dir="ltr"><div>Thank you so much for your reply!</div><div><br></div><div>I want my pass to run as late as possible to not be messed up by any of the regular internal optimizations. I need a reference of i32 %b (regularly we see this in IR as i32* %b) so if I understand you I think I need an indirection.</div><div>In that case is it safe to just create an AllocaInst ? Does adding the AllocaInst effect the way the data flows between functions?</div><div><br></div><div>The reason I am saying this is: Following my above example code, assuming I am interested in the local variable "m" declared and set in main() (so a memory slot dedicated for "m" is created in the stack, right?), "m" is passed to foo(m), nothing is altered, "m" is further passed to bar().</div><div>If I created the AllocaInst in bar and assign %b to it, will I be creating a new stack memory slot for bar's Argument "b" and will then I be passing in the memory address of "b" (context of bar()) and not "m" (context of main()).<br></div><div><br></div><div>I want to, if possible, pass the stack address of "m" (from the context of main) to my profiling function call that is currently in the context of bar(). <br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Feb 23, 2021 at 6:19 PM Johannes Doerfert <<a href="mailto:johannesdoerfert@gmail.com">johannesdoerfert@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">If you want to access the `llvm::AllocaInst` generated by Clang for<br>
the argument you should run your pass early in the pipeline. If you<br>
just want to see the value passed to `bar`, take the `llvm::Argument`<br>
and pass it to your profiling function. If you need indirection for<br>
some reason, create a new `llvm::AllocaInst`, store the `llvm::Argument`<br>
you are interested in in it, and pass the `llvm::AllocaInst` to your<br>
function.<br>
<br>
I hope this helps.<br>
<br>
~ Johannes<br>
<br>
<br>
On 2/23/21 3:19 PM, K Jelesnianski via llvm-dev wrote:<br>
> Dear LLVM mailing list,<br>
><br>
> I am currently having difficulty with figuring out how to get a function<br>
> parameter Value* when I am compiling with -O2. It seems -O2 performs an<br>
> optimization that optimizes out the local variable formed for the function<br>
> parameter.<br>
> I have a pass where I am inserting new function calls, and need to pass the<br>
> function parameters variable address. With -O0 this issue does not come up<br>
> as each function parameter when using -O0 gets its own AllocaInst and I can<br>
> leverage that Value* to pass to my new function call. I have tried to<br>
> leverage llvm.dbg information but it seems LLVM is not inserting<br>
> llvm.dbg.addr for me to use.<br>
><br>
> Below is the code example I am working with:<br>
> /* example.c */<br>
> int fd_in;<br>
> char* ut_addr;<br>
><br>
> __attribute__((noinline)) void bar (int b){<br>
> my_pass_function( &b ); // NEW (what I want to be done by my pass)<br>
> ut_addr = (char*) mmap( 0, b, my_prot, MAP_PRIVATE, fd_in, 0);<br>
> }<br>
><br>
> __attribute__((noinline)) void foo (int f){<br>
> bar( f );<br>
> }<br>
><br>
> int main(int argc, char** argv){<br>
> int m;<br>
> struct stat statbuf;<br>
> char *input;<br>
><br>
> if( (fd_in = open ( input, O_RDONLY)) < 0){ /* open the<br>
> input file */<br>
> return 0;<br>
> }<br>
> if( fstat( fd_in , &statbuf ) < 0 ){ /* find size of input<br>
> file */<br>
> return 0;<br>
> }<br>
><br>
> m = statbuf.st_size;<br>
> foo( m );<br>
> return 0;<br>
> }<br>
> -----------------------------------------------------------------<br>
> The original IR I get for bar() is below, as you can see the function<br>
> parameter is the "raw" value "i32 %b" passed into the function and not a<br>
> variable that I can leverage, get its Value* . Hence when I run my newly<br>
> instrumented code, I get a segmentation fault.<br>
><br>
> define void @bar(i32 %b) local_unnamed_addr #0 !dbg !35 {<br>
> entry:<br>
> call void @llvm.dbg.value(metadata i32 %b, metadata !39, metadata<br>
> !DIExpression()), !dbg !44<br>
> call void @llvm.dbg.value(metadata i64 1000, metadata !40, metadata<br>
> !DIExpression()), !dbg !44<br>
> call void @llvm.dbg.value(metadata i32 1, metadata !43, metadata<br>
> !DIExpression()), !dbg !44<br>
> %conv = sext i32 %b to i64, !dbg !45<br>
> %0 = load i32, i32* @fd_in, align 4, !dbg !46, !tbaa !47<br>
> %call = tail call i8* @mmap(i8* null, i64 %conv, i32 1, i32 2, i32 %0,<br>
> i64 0) #7, !dbg !51<br>
> store i8* %call, i8** @ut_addr, align 8, !dbg !52, !tbaa !53<br>
> ret void, !dbg !55<br>
> }<br>
><br>
><br>
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</blockquote></div>