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Change the cap on the amount of padding for each vtable to 32-byte (previously it was 128-byte)

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We tested different cap values with a recent commit of Chromium. Our results show that the 32-byte cap yields the smallest binary and all the caps yield similar performance.
Based on the results, we propose to change the cap value to 32-byte.

Patch by Zhaomo Yang!

Differential Revision: https://reviews.llvm.org/D49405

llvm-svn: 337622
This commit is contained in:
Peter Collingbourne 2018-07-20 21:43:20 +00:00
parent f463eb1647
commit 5c2365311a
1 changed files with 6 additions and 4 deletions
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10
lib/Transforms/IPO/LowerTypeTests.cpp
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@ -771,10 +771,12 @@ void LowerTypeTestsModule::buildBitSetsFromGlobalVariables(
// Compute the amount of padding required.
uint64_t Padding = NextPowerOf2(InitSize - 1) - InitSize;
// Cap at 128 was found experimentally to have a good data/instruction
// overhead tradeoff.
if (Padding > 128)
Padding = alignTo(InitSize, 128) - InitSize;
// Experiments of different caps with Chromium on both x64 and ARM64
// have shown that the 32-byte cap generates the smallest binary on
// both platforms while different caps yield similar performance.
// (see https://lists.llvm.org/pipermail/llvm-dev/2018-July/124694.html)
if (Padding > 32)
Padding = alignTo(InitSize, 32) - InitSize;
GlobalInits.push_back(
ConstantAggregateZero::get(ArrayType::get(Int8Ty, Padding)));