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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">If I understood it correctly, __declspec(noalias) is not the same as specifying restrict on each parameter.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">It means in the mentioned example a, b & c don't modify or reference any global state, but they are free to alias one another.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">You could specify restrict on each one to indicate that they do not alias each other.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">For more details refer:
<a href="https://msdn.microsoft.com/en-us/library/k649tyc7.aspx">https://msdn.microsoft.com/en-us/library/k649tyc7.aspx</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Ashutosh<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Phil Tomson via llvm-dev<br>
<b>Sent:</b> Friday, November 18, 2016 12:23 AM<br>
<b>To:</b> LLVM Developers Mailing List <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] Loop invariant not being optimized<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">I've got an example where I think that there should be some loop-invariant optimization happening, but it's not. Here's the C code:<br>
<br>
<span style="font-family:"Courier New"">#define DIM 8<br>
#define UNROLL_DIM DIM<br>
typedef double InArray[DIM][DIM];<br>
<br>
__declspec(noalias) void f1( InArray c, const InArray a, const InArray b )<br>
{<br>
<br>
#pragma clang loop unroll_count(UNROLL_DIM)<br>
for( int i=0;i<DIM;i++)<br>
#pragma clang loop unroll_count(UNROLL_DIM)<br>
for( int j=0;j<DIM;j++)<br>
#pragma clang loop unroll_count(UNROLL_DIM)<br>
for( int k=0;k<DIM;k++) {<br>
c[i][k] = c[i][k] + a[i][j]*b[j][k];<br>
}<br>
}</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">The "a[i][j]" there is invariant in that inner loop. I've unrolled the loops with the unroll pragma to make the assembly easier to read, here's what I see (LVM 3.9, compiling with: clang -fms-compatibility -funroll-loops
-O3 -c fma.c -o fma.o )<br>
<br>
<br>
<span style="font-family:"Courier New"">0000000000000000 <f1>:<br>
0: 29580c0000000000 load r3,r0,0x0,64<br>
8: 2958100200000000 load r4,r1,0x0,64 #r4 <- a[0][0]<br>
10: 2958140400000000 load r5,r2,0x0,64<br>
18: c0580c0805018000 fmaf r3,r4,r5,r3,64<br>
20: 79b80c0000000000 store r3,r0,0x0,64<br>
28: 2958100000000008 load r4,r0,0x8,64<br>
30: 2958140200000000 load r5,r1,0x0,64 #r5 <- a[0][0]<br>
38: 2958180400000008 load r6,r2,0x8,64<br>
40: c058100a06020000 fmaf r4,r5,r6,r4,64<br>
48: 79b8100000000008 store r4,r0,0x8,64<br>
50: 2958140000000010 load r5,r0,0x10,64<br>
58: 2958180200000000 load r6,r1,0x0,64 #r6 <- a[0][0]<br>
60: 29581c0400000010 load r7,r2,0x10,64<br>
68: c058140c07028000 fmaf r5,r6,r7,r5,64<br>
70: 79b8140000000010 store r5,r0,0x10,64<br>
78: 2958180000000018 load r6,r0,0x18,64<br>
80: 29581c0200000000 load r7,r1,0x0,64 #r7 <- a[0][0]<br>
88: 2958200400000018 load r8,r2,0x18,64<br>
90: c058180e08030000 fmaf r6,r7,r8,r6,64<br>
... </span><o:p></o:p></p>
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<p class="MsoNormal">(fmaf semantics are: fmaf r1,r2,r3,r4, SIZE r1 <- r2*r3+r4 )<o:p></o:p></p>
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<p class="MsoNormal">(load semantics are: load r1,r2,imm, SIZE r1<- mem[r2+imm] )<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">All three of the addresses are loaded in every loop. Only two need to be reloaded in the inner loop. I added the 'noalias' declspec in the C code above thinking that it would indicate that the pointers going
into the function are not aliased and that that would allow the optimization, but it didn't make any difference.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Of course it's easy to rewrite the example code to avoid this extra load/inner loop, but I would have thought this would be a fairly straighforward optimization for the optimizer. Am I missing something?<o:p></o:p></p>
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<p class="MsoNormal">Phil<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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