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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Unnecessary register spilling"
href="https://bugs.llvm.org/show_bug.cgi?id=38656">38656</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Unnecessary register spilling
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>maarten.bosmans@vortech.nl
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Clang compiles the loop in this function
void stencil1(int start, int stop, ptrdiff_t stride,
float *restrict a, float *restrict b,
float (*restrict c)[stride]) {
const float *restrict c1 = &c[1][0];
const float *restrict c2 = &c[2][0];
const float *restrict c3 = &c[3][0];
const float *restrict c4 = &c[4][0];
const float *restrict c5 = &c[5][0];
const float *restrict c6 = &c[6][0];
const float *restrict c7 = &c[7][0];
const float *restrict c8 = &c[8][0];
for (int i = start; i <= stop; i++) {
a[i] += b[1] * c1[i] + b[2] * c2[i]
+ b[3] * c3[i] + b[4] * c4[i]
+ b[5] * c5[i] + b[6] * c6[i]
+ b[7] * c7[i] + b[8] * c8[i];
}
}
as (using AVX2)
.LBB0_6: # =>This Inner Loop Header: Depth=1
lea rbx, [r11 + r13]
vmulps ymm0, ymm8, ymmword ptr [r10 + 4*r11]
mov r14, qword ptr [rsp - 88] # 8-byte Reload
vfmadd231ps ymm0, ymm9, ymmword ptr [r14 + 4*rbx] # ymm0 = (ymm9 * mem) +
ymm0
mov rax, qword ptr [rsp - 96] # 8-byte Reload
vfmadd231ps ymm0, ymm10, ymmword ptr [rax + 4*rbx] # ymm0 = (ymm10 * mem) +
ymm0
vfmadd231ps ymm0, ymm11, ymmword ptr [r8 + 4*rbx] # ymm0 = (ymm11 * mem) +
ymm0
mov rax, qword ptr [rsp - 104] # 8-byte Reload
vfmadd231ps ymm0, ymm12, ymmword ptr [rax + 4*rbx] # ymm0 = (ymm12 * mem) +
ymm0
vfmadd231ps ymm0, ymm13, ymmword ptr [r12 + 4*rbx] # ymm0 = (ymm13 * mem) +
ymm0
vfmadd231ps ymm0, ymm14, ymmword ptr [r15 + 4*rbx] # ymm0 = (ymm14 * mem) +
ymm0
vfmadd231ps ymm0, ymm15, ymmword ptr [rdi + 4*rbx] # ymm0 = (ymm15 * mem) +
ymm0
vaddps ymm0, ymm0, ymmword ptr [r9 + 4*r11]
vmovups ymmword ptr [r9 + 4*r11], ymm0
add r11, 8
cmp rbp, r11
jne .LBB0_6
The b values are broadcasted to ymm8-ymm15 before the loop, which is nice.
The same is not done for all the adresses of c1..c8. Some of them are stored in
registers, but others are loaded from the stack first in rax (and weirdly r14).
I think this harms performance. There should be enough registers free to hoist
the loading of the addresses outside the loop so the three mov instructions can
be removed from the loop.
If the c1..c8 variables are pointer arguments to a function instead of coming
out of a VLA calculation, the register spilling does not occur.
Godbolt link:
<a href="https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAKxAEZSAbAQwDtQB9T5UgZ1QFdiyTCADkAUgBMAZjwtkDflgDU46QGEeBdFgBmAOgRrs4gAwBBM%2BYBuqPOmVbM8vA1oQ5BRwSbECpZU9vVAAHAJCCYnxdXXYvLWJ7TFIrZTT0tN0GVCYvACpiTAS8ZC8mAKyc/MLi0uUAIxSLDIzK3OUIAqLIkq9kAEpxAFYAIWKsYYARftUAdhGrVPS0Fi1lNuruxLrkWlVpSdVJADZkYZHaKfPTK%2BkF5uXUVa8N5S7avsl9w6lT88krqMbkNJmp7uYMis1q93j0dtJvkc/qNpICRsDQXclmkoS9su1Yds%2BgAWRG/M6jYlojFg7HKXHrfGbD70oZkk4UkZDam3cGQp7QplvGpwvrHdnIkbHHkg2kPHECvFVYVbXr02YSzmzGWYvmPZ6M5WEtXIAAcmvOpp1css8vWJA6QTwiK0vgIYMC%2B3UakOWlCHrwUgWkhGM3E8zpGSY50DIKOCwODXOlzjeXpKZGsZ%2BIaTowBqfp%2BczVztLRaQdzI1RBeQ1eLcYr9XOVJrLfroNLZfSjfO3Jrffb8cr0prI/bka7aR7o21Ndng%2BnIytNeX46xdvDHcssy3VldBBKyls9m8zmQrkk9Q8LHiPj8ASCfrCE8yQuNdXKhoJIqJDSaEK7BkYR/E16HpRUv2ZUVC3/ScgLfED4QCeCjUQklYMAiDgNVHYhmQrCEJwsUMLLFDvyI9V8INbCWTNMMIwsOldAda8vGdH1vDdAMvQ4p8AyDIN6L1Mto1GLN4w4ptRhTQ4012GMpiHKSRnzWTCwU2VO0nRdqzU2sNOzEZKxbPS2yzF8u0XPs9IHcTFxHPSx3MrTLJzZTZz0%2Bc7Lcy1FLk1dzPXACpx3RZQosY8HCcFwGEkJhWM4%2B9AhvYIwmUCIojwGI4m8RIsBIloaOgz8it/RoLNeTo0PpQZRnGTApiEulovPWKr1dJKnwCT9ypc8lkx1AJ%2BrzQaLOGqtBqRTkTOuKYCoycbrNmkEho5c4HOWyZ5u7NaZ0m8bl1BIFGptTcrFEfpGDEIZRFIFgxFMW7UDEdRHAEIRMCOaRaFuggHouy6AGsQEkYl9GJIZjgATlNIZiWOU1TFNSQockK7RGJW77tER7SGe0Rbp4EBTFIP6cYu0g4FgJA0AAWxCVxMDICgIDphmGCZlBgGR0hdFcAgmaJiB6n%2B0h6jkXwAE8xB%2B0g6dp5wCAAeRYBhpfJ0gsFp1hgA50X8EKUo8GsIpRcwAAPTBkH4AWZduzxMAYUWelp/7LuYNgQE4dhuAYPB6iJyBLtCA8BTEABaLR0B9ZAoah5Rw4AdSYBgGATpXdCYNZw%2B1oQjAOBAs4Ad0dtPw90FhUHD/gWGIVBU/D7JQh4BOK6ruQ/ZYTBCfe4Q6Hd66sdF/HzdNY5w%2BOUlgGQZBlFNfQvggXBCAdKRvoCdRUHpxniC%2B2gZnUX63cuhBMCYLBiEoIGQekcHTWJWhpGOWgofh2ZTVmWZjnRzHSFd2hTAk2xrjfGhNiakzdpTGAiAUBb3ZkzcglA2Y7xQAoHWcRiA10BrzfmgtKAiw1uLFgUs7Zyy3grG8Ks1b60wNrNgesNYGytgeE2RMNYWytjbEQohZYOydhrF2kCPYcC4Iwf2gcIDBwiHgMOohI7aBjnHBOydU7p0ztnXOyB86TELjwEuqjy6V2rrXeuDBG6oGbq3IxHc5Dd14L3EQ%2B90Y3TusPMQo9x6T3pMIjokQsEzCXvgIgu815gU3tvDmISZDSAPkfcm/QT5nwvlfUgwMTj6CGLMYkccEamEfqDYkaN%2BG/3/oA1xGtQG8HAWTR6l0qYwOQZExBrM4EoPPDPWgpocEMAFsQIWBDcZEJITw268tFZUPVrjLWOsGFTLwIbFhpt2GW2trbEZ5AbyO2dokV28TGA6y9qIv2Ad4BSNDqsCOGcs5eBzr4LRPpdH6LLjXOuqcfS4z4IIPuTj%2BEuOAU9dxY8J6khKLPTp%2BhTAQo6MvYJe8N6tMiV9IYsSIHxMSefTmkjUkgyGHfaQppTS0CGNIIYUNH6mihj/W6pSgFuIJlUkmNSAbo0kEPCpYg4m1MuibPpMj7rEiAA%3D%3D">https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAKxAEZSAbAQwDtQB9T5UgZ1QFdiyTCADkAUgBMAZjwtkDflgDU46QGEeBdFgBmAOgRrs4gAwBBM%2BYBuqPOmVbM8vA1oQ5BRwSbECpZU9vVAAHAJCCYnxdXXYvLWJ7TFIrZTT0tN0GVCYvACpiTAS8ZC8mAKyc/MLi0uUAIxSLDIzK3OUIAqLIkq9kAEpxAFYAIWKsYYARftUAdhGrVPS0Fi1lNuruxLrkWlVpSdVJADZkYZHaKfPTK%2BkF5uXUVa8N5S7avsl9w6lT88krqMbkNJmp7uYMis1q93j0dtJvkc/qNpICRsDQXclmkoS9su1Yds%2BgAWRG/M6jYlojFg7HKXHrfGbD70oZkk4UkZDam3cGQp7QplvGpwvrHdnIkbHHkg2kPHECvFVYVbXr02YSzmzGWYvmPZ6M5WEtXIAAcmvOpp1css8vWJA6QTwiK0vgIYMC%2B3UakOWlCHrwUgWkhGM3E8zpGSY50DIKOCwODXOlzjeXpKZGsZ%2BIaTowBqfp%2BczVztLRaQdzI1RBeQ1eLcYr9XOVJrLfroNLZfSjfO3Jrffb8cr0prI/bka7aR7o21Ndng%2BnIytNeX46xdvDHcssy3VldBBKyls9m8zmQrkk9Q8LHiPj8ASCfrCE8yQuNdXKhoJIqJDSaEK7BkYR/E16HpRUv2ZUVC3/ScgLfED4QCeCjUQklYMAiDgNVHYhmQrCEJwsUMLLFDvyI9V8INbCWTNMMIwsOldAda8vGdH1vDdAMvQ4p8AyDIN6L1Mto1GLN4w4ptRhTQ4012GMpiHKSRnzWTCwU2VO0nRdqzU2sNOzEZKxbPS2yzF8u0XPs9IHcTFxHPSx3MrTLJzZTZz0%2Bc7Lcy1FLk1dzPXACpx3RZQosY8HCcFwGEkJhWM4%2B9AhvYIwmUCIojwGI4m8RIsBIloaOgz8it/RoLNeTo0PpQZRnGTApiEulovPWKr1dJKnwCT9ypc8lkx1AJ%2BrzQaLOGqtBqRTkTOuKYCoycbrNmkEho5c4HOWyZ5u7NaZ0m8bl1BIFGptTcrFEfpGDEIZRFIFgxFMW7UDEdRHAEIRMCOaRaFuggHouy6AGsQEkYl9GJIZjgATlNIZiWOU1TFNSQockK7RGJW77tER7SGe0Rbp4EBTFIP6cYu0g4FgJA0AAWxCVxMDICgIDphmGCZlBgGR0hdFcAgmaJiB6n%2B0h6jkXwAE8xB%2B0g6dp5wCAAeRYBhpfJ0gsFp1hgA50X8EKUo8GsIpRcwAAPTBkH4AWZduzxMAYUWelp/7LuYNgQE4dhuAYPB6iJyBLtCA8BTEABaLR0B9ZAoah5Rw4AdSYBgGATpXdCYNZw%2B1oQjAOBAs4Ad0dtPw90FhUHD/gWGIVBU/D7JQh4BOK6ruQ/ZYTBCfe4Q6Hd66sdF/HzdNY5w%2BOUlgGQZBlFNfQvggXBCAdKRvoCdRUHpxniC%2B2gZnUX63cuhBMCYLBiEoIGQekcHTWJWhpGOWgofh2ZTVmWZjnRzHSFd2hTAk2xrjfGhNiakzdpTGAiAUBb3ZkzcglA2Y7xQAoHWcRiA10BrzfmgtKAiw1uLFgUs7Zyy3grG8Ks1b60wNrNgesNYGytgeE2RMNYWytjbEQohZYOydhrF2kCPYcC4Iwf2gcIDBwiHgMOohI7aBjnHBOydU7p0ztnXOyB86TELjwEuqjy6V2rrXeuDBG6oGbq3IxHc5Dd14L3EQ%2B90Y3TusPMQo9x6T3pMIjokQsEzCXvgIgu815gU3tvDmISZDSAPkfcm/QT5nwvlfUgwMTj6CGLMYkccEamEfqDYkaN%2BG/3/oA1xGtQG8HAWTR6l0qYwOQZExBrM4EoPPDPWgpocEMAFsQIWBDcZEJITw268tFZUPVrjLWOsGFTLwIbFhpt2GW2trbEZ5AbyO2dokV28TGA6y9qIv2Ad4BSNDqsCOGcs5eBzr4LRPpdH6LLjXOuqcfS4z4IIPuTj%2BEuOAU9dxY8J6khKLPTp%2BhTAQo6MvYJe8N6tMiV9IYsSIHxMSefTmkjUkgyGHfaQppTS0CGNIIYUNH6mihj/W6pSgFuIJlUkmNSAbo0kEPCpYg4m1MuibPpMj7rEiAA%3D%3D</a></pre>
</div>
</p>
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