<div dir="ltr"><br><div class="gmail_extra"><br><br><div class="gmail_quote">On Fri, Oct 11, 2013 at 12:06 AM, John Criswell <span dir="ltr"><<a href="mailto:criswell@illinois.edu" target="_blank">criswell@illinois.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div class="im">On 10/10/13 10:43 AM, Hongxu Chen 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"><div class="im">
Hi, this question might be a bit silly: apart from the language<br>
reference(<a href="http://llvm.org/docs/LangRef.html#switch-instruction" target="_blank">http://llvm.org/<u></u>docs/LangRef.html#switch-<u></u>instruction</a>) page, are<br>
there additional rules for a regular llvm frontend to generate llvm IRs?<br>
<br>
There are a few cases that I got from clang/llvm-gcc/dragonegg when<br></div>
compiling *C* source code into llvm IR:<div class="im"><br>
<br>
1. It seems that there is ONLY ONE ReturnInst(and NO InvokeInst) for such<br></div>
llvm IR; is it legal to add other *ReturnInst*s when transforming?<br>
</blockquote>
<br>
An LLVM function can have multiple ReturnInsts as long as each one terminates a basic block. There is a transform (UnifyExitNodes, IIRC) that will take a function with multiple ReturnInsts and create one with a single ReturnInst. Having a single ReturnInst (exit node) simplifies other analyses.<div class="im">
<br></div></blockquote><div>Thanks so much, John; especially for pointing out 'UnifyExitNodes' pass! <br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div class="im">
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
2. Is it possible for a frontend to generate a function whose CFG is<br>
something like:<br>
<br>
bb0<br>
/ \<br>
bb1 bb2<br>
/ \ / \<br>
bb3 bb4 bb5<br>
\ | /<br>
\ | /<br>
\ | /<br>
bb6<br>
<br>
(In this case, if I understand correctly, bb4 is control dependent on both<br>
bb1 and bb2.)<br>
I think it at least possible in theory, and there is a simple case:<br>
</blockquote>
<br></div>
Yes, that looks fine to me. One of the LLVM passes might optimize that CFG or put it into some canonical form, but that CFG looks fine to me.<br>
<br>
-- John T.<br></blockquote><div><br></div><div>Got it!<br><br></div><div>And can I say : as long as it is not explicitly in llvm language reference, there are generally no restrictions for frontends/transformations to generate IR(of course, they pass the verifier)?<br>
</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div class="im">
<br>
int foo(int i) {<br>
if (i < 0) {<br>
if (i % 2 == 0) {<br>
i += 1;<br>
} else {<br>
i += 2;<br>
}<br>
} else {<br>
if (i % 2 == 0) {<br>
i += 1;<br>
} else {<br>
i += 2;<br>
}<br>
}<br>
return 0;<br>
}<br>
<br>
However none of the frontends I used generate the basicblocks like<br></div>
that(there is always one or more basicblocks generated) /without any<br>
optimizations/. So is there any implicit rules for these frontends?<div class="im"><br>
<br>
And can I rely on these cases when I ONLY deal with C source code?<br>
<br>
Thanks!<br>
<br>
<br>
<br></div><span class=""><font color="#888888">
--<br>
View this message in context: <a href="http://llvm.1065342.n5.nabble.com/Are-there-implicit-rules-or-conventions-for-an-llvm-frontend-to-generate-llvm-IR-tp61938.html" target="_blank">http://llvm.1065342.n5.nabble.<u></u>com/Are-there-implicit-rules-<u></u>or-conventions-for-an-llvm-<u></u>frontend-to-generate-llvm-IR-<u></u>tp61938.html</a><br>
Sent from the LLVM - Dev mailing list archive at Nabble.com.<br>
______________________________<u></u>_________________<br>
LLVM Developers mailing list<br>
<a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu" target="_blank">http://llvm.cs.uiuc.edu</a><br>
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/<u></u>mailman/listinfo/llvmdev</a><br>
</font></span></blockquote>
<br>
</blockquote></div><br></div></div>