Hi James,<div><br></div><div>Thank you very much for the explanation .</div><div><br></div><div>FYI .LLVM Dead store elimination (DSE) transformation eliminates those extra bytes. i.e </div><div><br></div><div><div>$ opt -f -S -dse enum.s</div>
<div>; ModuleID = 'enum.s'</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> %s = alloca i32, align 4</div><div> store i32 1, i32* %s, align 4</div><div> %1 = load i32* %s, align 4</div><div> ret i32 %1</div><div>}</div><div><br></div>
<div>Duncan, Debugging the Clang source code here for the reason "why these extra bytes",But still you have anything in your kit,Please let me know :). </div><div><br></div><div>Thanks </div><div>~Umesh </div>
<div><br></div><br><div class="gmail_quote">On Thu, Feb 2, 2012 at 2:54 PM, James Molloy <span dir="ltr"><<a href="mailto:James.Molloy@arm.com">James.Molloy@arm.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Hi Umesh,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">The compiler generates an alloca (stack space) for every local variable defined in a function. Along with local variables this also includes space to store
the function parameters and also the variable that holds the return value.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">For example:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Int x(int a, int b) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> Return a + b;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Would compile to something like this:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Define i32 @x(i32 %a, i32 %b) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %ret.addr = alloca i32<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %a.addr = alloca i32<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %b.addr = alloca i32<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> Store i32 %a, i32 *%a.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> Store i32 %b, i32 *%b.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %1 = load i32* %a.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %2 = load i32* %b.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %3 = add nsw i32 %1, %2<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> Store i32 %3, i32* %ret.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> %4 = load i32* %ret.addr<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> Ret %4<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">So firstly Clang would create variables on the stack to hold the parameters and the return value, then store the parameters there (function prologue).<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Then, to perform the add, it would load the arguments from their local variable storage (in this case %a.addr and %b.addr), perform the add and write the
result back to its local variable storage (%ret.addr)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Finally to return it will load the return value variable and return it.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Does this make things clearer?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Cheers,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">James<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> <a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a> [mailto:<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a>]
<b>On Behalf Of </b>Umesh Kalappa<br>
<b>Sent:</b> 02 February 2012 09:04<br>
<b>To:</b> Duncan Sands<br>
<b>Cc:</b> <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [LLVMdev] (no subject)<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Hi Duncan,<u></u><u></u></p>
<div>
<p class="MsoNormal">Appreciate you response here and yeah any optimization switch for clang will make this extra bytes go away ...Was very curious to know why these extra bytes for and can you please elaborate more on "for storing the return value in<u></u><u></u></p>
</div>
<p class="MsoNormal">more complicated cases" ...That helps me understand the LLVM internals :) <u></u><u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal">Thanks <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">~Umesh <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><u></u> <u></u></p>
<div>
<p class="MsoNormal">On Thu, Feb 2, 2012 at 2:19 PM, Duncan Sands <<a href="mailto:baldrick@free.fr" target="_blank">baldrick@free.fr</a>> wrote:<u></u><u></u></p>
<p class="MsoNormal">Hi Umesh,<u></u><u></u></p>
<div>
<p class="MsoNormal"><br>
> Again ,I'm newbie to LLVM and please pardon me ..if you guys feel that ,the<br>
> below question is very basic :)<br>
><u></u><u></u></p>
</div>
<p class="MsoNormal">> Here i go ,compiled the below sample with clang i.e *clang enum.c -S -emit-llvm*<u></u><u></u></p>
<div>
<p class="MsoNormal">> and there respective file are<br>
><br>
> $ cat enum.c<br>
> int main()<br>
> {<br>
> enum type{one=1,two,three} s;<br>
> s = one;<br>
> return s;<br>
> }<br>
><br>
> $ cat enum.s<br>
> ; ModuleID = 'enum.c'<br>
> target datalayout =<br>
> "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"<br>
> target triple = "i386-pc-cygwin"<br>
><br>
> define i32 @main() nounwind {<br>
> %1 = alloca i32, align 4<br>
> %s = alloca i32, align 4<br>
> store i32 0, i32* %1<br>
> store i32 1, i32* %s, align 4<br>
> %2 = load i32* %s, align 4<br>
> ret i32 %2<br>
> }<br>
><u></u><u></u></p>
</div>
<p class="MsoNormal">> *Question :* Why there is extra 4 bytes on stack i.e *"%1 = alloca i32, align<br>
> 4"* ???<br>
<br>
I think this would normally be used for storing the return value in more<br>
complicated cases. If you compile with optimization it will go away.<br>
<br>
Ciao, Duncan.<br>
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