<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><div> ; push foo (On "stack")</div><div> %sp_1 = getelementptr i8, i8* %sp, i32 -4</div><div> %sp_1_ptr = bitcast i8* %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 = getelementptr i8, i8* %sp_1, i32 -4</div><div> %sp_2_ptr = bitcast i8* %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 = bitcast i8* %sp_2 to i32*</div><div> %val1 = load i32, i32* %sp_3_ptr, align 4</div><div> %sp_3 = getelementptr i8, i8* %sp_2, i32 4</div><div><br></div><div> ; val2 = pop (val2 = foo)</div><div> %sp_4_ptr = bitcast i8* %sp_3 to i32*</div><div> %val2 = load i32, i32* %sp_4_ptr, align 4</div><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><div> store i32 %foo, i32* %sp_1_ptr, align 4</div><div> %sp_2 = getelementptr i8, i8* %sp, i64 -8</div><div> %sp_2_ptr = bitcast i8* %sp_2 to i32*</div><div> store i32 %bar, i32* %sp_2_ptr, align 4</div><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>- Might it be possible to get inttoptr providing such alias analysis ?</div><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>- 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>Regards,</div><div>Paul</div><div><br></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:0 0 0 .8ex;border-left:1px #ccc 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 class="h5"><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 class="h5">
<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>
</div></div><pre>_______________________________________________
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</blockquote>
<br>
</div>
</blockquote></div><br></div>