[llvm] 7647c27 - [SimpleLoopUnswitch][NFC] Add option to always drop make.implicit metadata in non-trivial unswitching and save compile time

[Max Kazantsev via llvm-commits]

Author: Max Kazantsev
Date: 2020-08-04T10:16:40+07:00
New Revision: 7647c2716e383c091b7063e150d48d5821bcaa67

URL: https://github.com/llvm/llvm-project/commit/7647c2716e383c091b7063e150d48d5821bcaa67
DIFF: https://github.com/llvm/llvm-project/commit/7647c2716e383c091b7063e150d48d5821bcaa67.diff

LOG: [SimpleLoopUnswitch][NFC] Add option to always drop make.implicit metadata in non-trivial unswitching and save compile time

We might want this if we find out that using of MustExecute analysis is too expensive.
By default we do the analysis because its complexity does not exceed the complexity
of whole loop copying in unswitching. Follow-up for D84925.

Differential Revision: https://reviews.llvm.org/D85001
Reviewed By: asbirlea

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index b1cbc714ec8e..ab1945a1aff1 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -94,6 +94,11 @@ static cl::opt<bool> UnswitchGuards(
     "simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
     cl::desc("If enabled, simple loop unswitching will also consider "
              "llvm.experimental.guard intrinsics as unswitch candidates."));
+static cl::opt<bool> DropNonTrivialImplicitNullChecks(
+    "simple-loop-unswitch-drop-non-trivial-implicit-null-checks",
+    cl::init(false), cl::Hidden,
+    cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
+             "null checks to save time analyzing if we can keep it."));
 
 /// Collect all of the loop invariant input values transitively used by the
 /// homogeneous instruction graph from a given root.
@@ -2074,12 +2079,18 @@ static void unswitchNontrivialInvariants(
   // Drop metadata if we may break its semantics by moving this instr into the
   // split block.
   if (TI.getMetadata(LLVMContext::MD_make_implicit)) {
-    // It is only legal to preserve make.implicit metadata if we are guaranteed
-    // to reach implicit null check block after following this branch.
-    ICFLoopSafetyInfo SafetyInfo;
-    SafetyInfo.computeLoopSafetyInfo(&L);
-    if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
+    if (DropNonTrivialImplicitNullChecks)
+      // Do not spend time trying to understand if we can keep it, just drop it
+      // to save compile time.
       TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
+    else {
+      // It is only legal to preserve make.implicit metadata if we are
+      // guaranteed no reach implicit null check after following this branch.
+      ICFLoopSafetyInfo SafetyInfo;
+      SafetyInfo.computeLoopSafetyInfo(&L);
+      if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
+        TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
+    }
   }
 
   // The stitching of the branched code back together depends on whether we're