<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">Hi again,</div><div class="gmail_quote"><br></div><div class="gmail_quote">So I finally gave up on trying to get through the converting (x86' push pop mov add) because it deals a lot with crazy pointer arithmetics and sonce inttoptr and ptrtoint doesn't provide any alias analysis information.</div><div class="gmail_quote">Daniel, you said it doesn't make much sense to provide it but in my cases it is actually very much needed, you didn't say it wasn't possible to provide it but it is possible right? Could you guide me through where I should look to implement such analysis? Would it be complex/non-trivial to implement such thing?</div><div class="gmail_quote"><br></div><div class="gmail_quote">2016-02-10 21:24 GMT+01:00 Daniel Berlin <span dir="ltr"><<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>></span>:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="h5">On Wed, Feb 10, 2016 at 12:18 PM, Paul Peet via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">Thank you for the hint.<div><br></div><div>I adjusted the code and it works:</div><div><br></div><div>The code after replacing inttoptr with getelementptr:</div><div><br></div><div><div>define { i32, i32, i8* } @test(i32 %foo, i32 %bar, i8* %sp) {</div><div>entry:</div><span><div> ; push foo (On "stack")</div></span><div> %sp_1 = getelementptr i8, i8* %sp, i32 -4</div><div> %sp_1_ptr = bitcast i8* %sp_1 to i32*</div><span><div> store i32 %foo, i32* %sp_1_ptr, align 4</div><div><br></div><div> ; push bar</div></span><div> %sp_2 = getelementptr i8, i8* %sp_1, i32 -4</div><div> %sp_2_ptr = bitcast i8* %sp_2 to i32*</div><span><div> store i32 %bar, i32* %sp_2_ptr, align 4</div><div><br></div><div> ; val1 = pop (val1 = bar)</div></span><div> %sp_3_ptr = bitcast i8* %sp_2 to i32*</div><span><div> %val1 = load i32, i32* %sp_3_ptr, align 4</div></span><div> %sp_3 = getelementptr i8, i8* %sp_2, i32 4</div><span><div><br></div><div> ; val2 = pop (val2 = foo)</div></span><div> %sp_4_ptr = bitcast i8* %sp_3 to i32*</div><span><div> %val2 = load i32, i32* %sp_4_ptr, align 4</div></span><div> %sp_4 = getelementptr i8, i8* %sp_3, i32 4</div><div><br></div><div> %ret_1 = insertvalue { i32, i32, i8* } undef, i32 %val1, 0</div><div> %ret_2 = insertvalue { i32, i32, i8* } %ret_1, i32 %val2, 1</div><div> %ret_3 = insertvalue { i32, i32, i8* } %ret_2, i8* %sp_4, 2</div><div><br></div><div> ret { i32, i32, i8* } %ret_3</div><div>}</div></div><div><br></div><div>After optimization ("opt -instcombine ./code.ll -S")</div><div><br></div><div><div>define { i32, i32, i8* } @test(i32 %foo, i32 %bar, i8* %sp) {</div><div>entry:</div><div> %sp_1 = getelementptr i8, i8* %sp, i64 -4</div><div> %sp_1_ptr = bitcast i8* %sp_1 to i32*</div><span><div> store i32 %foo, i32* %sp_1_ptr, align 4</div></span><div> %sp_2 = getelementptr i8, i8* %sp, i64 -8</div><div> %sp_2_ptr = bitcast i8* %sp_2 to i32*</div><span><div> store i32 %bar, i32* %sp_2_ptr, align 4</div></span><div> %ret_1 = insertvalue { i32, i32, i8* } undef, i32 %bar, 0</div><div> %ret_2 = insertvalue { i32, i32, i8* } %ret_1, i32 %foo, 1</div><div> %ret_3 = insertvalue { i32, i32, i8* } %ret_2, i8* %sp, 2</div><div> ret { i32, i32, i8* } %ret_3</div><div>}</div></div><div><br></div><div>My only questions are now:</div><div>- How is it that inttoptr cannot provide that specific alias information so it can optimize that store/load away ?</div></div></blockquote></div></div><div>Because nothing tracks what happens to the ints, and what happens when they are converted back to pointers and whether it's sane :)</div><div> <a href="http://llvm.org/docs/GetElementPtr.html#how-is-gep-different-from-ptrtoint-arithmetic-and-inttoptr" target="_blank">http://llvm.org/docs/GetElementPtr.html#how-is-gep-different-from-ptrtoint-arithmetic-and-inttoptr</a></div><span class=""><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><div>- Might it be possible to get inttoptr providing such alias analysis ?</div></div></blockquote></span><div>It doesn't make a lot of sense to try in most cases.</div><div>Most of the cases ptrtoint/inttoptr is useful are those where you want to do crazy things to the pointer.</div><span class=""><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><div>- I came across MemorySSA while browsing though the llvm source. Is it possible that one can use MemorySSA to do such optimization without alias analysis ?</div></div></blockquote><div><br></div></span><div>MemorySSA relies on alias analysis to generate the SSA form.</div><span class=""><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><div>- Where do I have to look in the source which is doing this kind of optimization (Is it instcombine which uses lib/Analysis/Loads.cpp ?)</div><div><br></div></div></blockquote></span><div>It's probably a combination of opts. The most likely candidate is -gvn, but I would look at the pass dumps after each opt </div><div><div class="h5"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><div></div><div>Regards,</div><div>Paul</div><div><br></div></div><div><div><div class="gmail_extra"><br><div class="gmail_quote">2016-02-10 0:26 GMT+01:00 Philip Reames <span dir="ltr"><<a href="mailto:listmail@philipreames.com" target="_blank">listmail@philipreames.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
Two points:<br>
- Using inttoptr is a mistake here. GEPs are strongly preferred and
provide strictly more aliasing information to the optimizer.<br>
- The zext is a bit weird. I'm not sure where that came from, but
I'd not bother looking into until the preceding point is addressed.
<br>
<br>
In general, you may find these docs useful:<br>
<a href="http://llvm.org/docs/Frontend/PerformanceTips.html" target="_blank">http://llvm.org/docs/Frontend/PerformanceTips.html</a><br>
<br>
Philip<div><div><br>
<br>
<br>
<div>On 02/08/2016 06:54 AM, Paul Peet via
llvm-dev wrote:<br>
</div>
</div></div><blockquote type="cite"><div><div>
<div dir="ltr">Hello,
<div><br>
</div>
<div>I am trying to emulate the "stack" as like on x86 when
using push/pop so afterwards I can use LLVM's optimizer passes
to simplify (reduce junk) the code.</div>
<div><br>
</div>
<div>The LLVM IR code:</div>
<div><br>
</div>
<div>
<div>define { i32, i32, i32 } @test(i32 %foo, i32 %bar, i32
%sp) {</div>
<div> ; push foo (On "stack")</div>
<div> %sp_1 = sub i32 %sp, 4</div>
<div> %sp_1_ptr = inttoptr i32 %sp_1 to i32*</div>
<div> store i32 %foo, i32* %sp_1_ptr, align 4</div>
<div><br>
</div>
<div> ; push bar</div>
<div> %sp_2 = sub i32 %sp_1, 4</div>
<div> %sp_2_ptr = inttoptr i32 %sp_2 to i32*</div>
<div> store i32 %bar, i32* %sp_2_ptr, align 4</div>
<div><br>
</div>
<div> ; val1 = pop (val1 = bar)</div>
<div> %sp_3_ptr = inttoptr i32 %sp_2 to i32*</div>
<div> %val1 = load i32, i32* %sp_3_ptr, align 4</div>
<div> %sp_3 = add i32 %sp_2, 4</div>
<div><br>
</div>
<div> ; val2 = pop (val2 = foo)</div>
<div> %sp_4_ptr = inttoptr i32 %sp_3 to i32*</div>
<div> %val2 = load i32, i32* %sp_4_ptr, align 4</div>
<div> %sp_4 = add i32 %sp_3, 4</div>
<div><br>
</div>
<div> %ret_1 = insertvalue { i32, i32, i32 } undef, i32
%val1, 0</div>
<div> %ret_2 = insertvalue { i32, i32, i32 } %ret_1, i32
%val2, 1</div>
<div> %ret_3 = insertvalue { i32, i32, i32 } %ret_2, i32
%sp_4, 2</div>
<div><br>
</div>
<div> ret { i32, i32, i32 } %ret_3</div>
<div>}</div>
</div>
<div><br>
</div>
<div>This code will "push" two values onto the stack and pop
them in reverse order so afterwards "foo" and "bar" will be
swapped and returned back.</div>
<div><br>
</div>
<div>After running this through "opt -O2 ./test.ll", I am
getting this:</div>
<div><br>
</div>
<div>
<div>define { i32, i32, i32 } @test(i32 %foo, i32 %bar, i32
%sp) #0 {</div>
<div> %sp_1 = add i32 %sp, -4</div>
<div> %1 = zext i32 %sp_1 to i64</div>
<div> %sp_1_ptr = inttoptr i64 %1 to i32*</div>
<div> store i32 %foo, i32* %sp_1_ptr, align 4</div>
<div> %sp_2 = add i32 %sp, -8</div>
<div> %2 = zext i32 %sp_2 to i64</div>
<div> %sp_2_ptr = inttoptr i64 %2 to i32*</div>
<div> store i32 %bar, i32* %sp_2_ptr, align 4</div>
<div> %val2 = load i32, i32* %sp_1_ptr, align 4</div>
<div> %ret_1 = insertvalue { i32, i32, i32 } undef, i32 %bar,
0 ; Swapped</div>
<div> %ret_2 = insertvalue { i32, i32, i32 } %ret_1, i32
%val2, 1; Not Swapped (Not optimized; Should be %foo)</div>
<div> %ret_3 = insertvalue { i32, i32, i32 } %ret_2, i32 %sp,
2</div>
<div> ret { i32, i32, i32 } %ret_3</div>
<div>}</div>
</div>
<div><br>
</div>
<div>As you can see that the IR has got additional code, eg.
zext. But the main problem here is that val2 hasn't been
optimized.</div>
<div>Could anyone show me some hints what is preventing the
second val from being optimized? (My guess would be the zext
because I am using %sp as a 32bit pointer although the
"target" is 64bit).</div>
<div><br>
</div>
<div>Regards,</div>
<div>Paul</div>
</div>
<br>
<fieldset></fieldset>
<br>
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