[llvm] 5f187f0 - [SamplePGO] Add switch to honor zero count on block level as accurate

[Wenlei He via llvm-commits]

Author: Wenlei He
Date: 2021-09-21T17:06:37-07:00
New Revision: 5f187f0afaad33013ba03454c4749d99b1362534

URL: https://github.com/llvm/llvm-project/commit/5f187f0afaad33013ba03454c4749d99b1362534
DIFF: https://github.com/llvm/llvm-project/commit/5f187f0afaad33013ba03454c4749d99b1362534.diff

LOG: [SamplePGO] Add switch to honor zero count on block level as accurate

Add a new LLVM switch `-profile-sample-block-accurate` to trust zero block counts for branches. Currently we leave out such zero counts when annotating branch weight metadata, which would lead to weights being considered as unknown.

Differential Revision: https://reviews.llvm.org/D110117

Added: 
    

Modified: 
    llvm/lib/Transforms/IPO/SampleProfile.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/IPO/SampleProfile.cpp b/llvm/lib/Transforms/IPO/SampleProfile.cpp
index 663c29fe00cc2..23e3c94f9f288 100644
--- a/llvm/lib/Transforms/IPO/SampleProfile.cpp
+++ b/llvm/lib/Transforms/IPO/SampleProfile.cpp
@@ -143,6 +143,12 @@ static cl::opt<bool> ProfileSampleAccurate(
              "callsite and function as having 0 samples. Otherwise, treat "
              "un-sampled callsites and functions conservatively as unknown. "));
 
+static cl::opt<bool> ProfileSampleBlockAccurate(
+    "profile-sample-block-accurate", cl::Hidden, cl::init(false),
+    cl::desc("If the sample profile is accurate, we will mark all un-sampled "
+             "branches and calls as having 0 samples. Otherwise, treat "
+             "them conservatively as unknown. "));
+
 static cl::opt<bool> ProfileAccurateForSymsInList(
     "profile-accurate-for-symsinlist", cl::Hidden, cl::ZeroOrMore,
     cl::init(true),
@@ -1529,7 +1535,7 @@ void SampleProfileLoader::generateMDProfMetadata(Function &F) {
                             {static_cast<uint32_t>(BlockWeights[BB])}));
         }
       }
-    } else if (OverwriteExistingWeights) {
+    } else if (OverwriteExistingWeights || ProfileSampleBlockAccurate) {
       // Set profile metadata (possibly annotated by LTO prelink) to zero or
       // clear it for cold code.
       for (auto &I : BB->getInstList()) {