<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Feb 2, 2012, at 10:59 AM, Matthieu Monrocq wrote:</div><blockquote type="cite"><div class="gmail_quote">Le 2 février 2012 12:48, Umesh Kalappa <span dir="ltr"><<a href="mailto:umesh.kalappa0@gmail.com">umesh.kalappa0@gmail.com</a>></span> a écrit :<br><blockquote class="gmail_quote" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex; position: static; z-index: auto; ">
<div class="gmail_quote">Hi There ,<br><div class="gmail_quote"><div><br></div><div>I'm new to Clang and please pardon me ..if you guys feel that ,the below question is very basic :) </div><div><br></div><div>Here i go ,compiled the below sample with clang i.e <b>clang enum.c -S -emit-llvm</b> and respective files are </div>
<div><br></div><div><div>$ cat enum.c</div><div>int main()</div><div>{</div><div> enum type{one=1,two,three} s;</div><div> s = one;</div><div> return s;</div><div>}</div></div><div><br></div><div><div>$ cat enum.s</div><div>
; ModuleID = 'enum.c'</div><div>target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S32"</div><div>target triple = "i386-pc-cygwin"</div>
<div><br></div><div>define i32 @main() nounwind {</div><div> %1 = alloca i32, align 4</div><div> %s = alloca i32, align 4</div><div> store i32 0, i32* %1</div><div> store i32 1, i32* %s, align 4</div><div> %2 = load i32* %s, align 4</div>
<div> ret i32 %2</div><div>}</div></div><div><br></div><div><b>Question :</b> Why there is extra 4 bytes on stack i.e <b>"%1 = alloca i32, align 4"</b> ???</div><div><br></div><div>Note : Please note that the LLVM transformation will free away those extra space ,But still like to know why Clang at first shot introduces those extra spaces ?? </div>
<div><br></div></div></div></blockquote><div>It looks to me like %1 is allocated just in case we would need it for a "ret i32 %1" instruction at the end of main (since it's not mandatory for the user to define it). The "store i32 0, i32* %1" seems to agree with this understanding, as 0 is the default return value.<br></div></div></blockquote></div><br><div>Yes. We allocate an implicit local variable to hold the return value; return statements then just initialize the return slot and jump to the epilogue, where the slot is loaded and returned. We don't use a phi because the control flow for getting to the epilogue is not necessarily as simple as a simple branch, due to cleanups in local scopes (like C++ destructors).</div><div><br></div><div>Implicit return values like main's are handled with an implicit store in the prologue.</div><div><br></div><div>John.</div></body></html>