[PATCH] D49405: Change the cap on the amount of padding for each vtable to 32-byte (previously it was 128-byte)

[Zhaomo Yang via Phabricator via llvm-commits]

zhaomo updated this revision to Diff 156185.
zhaomo added a comment.

Added more information about the experiments in the comment.


https://reviews.llvm.org/D49405

Files:
  llvm/lib/Transforms/IPO/LowerTypeTests.cpp


Index: llvm/lib/Transforms/IPO/LowerTypeTests.cpp
===================================================================
--- llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -771,10 +771,12 @@
     // 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 now 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)));


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