[llvm] 2c7077e - [CodeGen] Split out cold exception handling pads.

[Snehasish Kumar via llvm-commits]

Author: Snehasish Kumar
Date: 2021-02-11T11:23:43-08:00
New Revision: 2c7077e67dc77834607c877ad6e8caa6e33ae238

URL: https://github.com/llvm/llvm-project/commit/2c7077e67dc77834607c877ad6e8caa6e33ae238
DIFF: https://github.com/llvm/llvm-project/commit/2c7077e67dc77834607c877ad6e8caa6e33ae238.diff

LOG: [CodeGen] Split out cold exception handling pads.

Support for splitting exception handling pads was added in D73739. This
change updates the code to split out exception handling pads if profile
information indicates that they are cold. For a given function with
multiple landind pads, if one of them is hot they are all retained as
part of the hot code section.

Differential Revision: https://reviews.llvm.org/D96372

Added: 
    

Modified: 
    llvm/lib/CodeGen/MachineFunctionSplitter.cpp
    llvm/test/CodeGen/X86/machine-function-splitter.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
index 483809a8ed96..294e3c273b00 100644
--- a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
+++ b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
@@ -23,6 +23,7 @@
 // https://groups.google.com/d/msg/llvm-dev/RUegaMg-iqc/wFAVxa6fCgAJ
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ProfileSummaryInfo.h"
 #include "llvm/CodeGen/BasicBlockSectionUtils.h"
@@ -77,7 +78,7 @@ class MachineFunctionSplitter : public MachineFunctionPass {
 };
 } // end anonymous namespace
 
-static bool isColdBlock(MachineBasicBlock &MBB,
+static bool isColdBlock(const MachineBasicBlock &MBB,
                         const MachineBlockFrequencyInfo *MBFI,
                         ProfileSummaryInfo *PSI) {
   Optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB);
@@ -121,16 +122,28 @@ bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
   auto *MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
   auto *PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
 
+  SmallVector<MachineBasicBlock *, 2> LandingPads;
   for (auto &MBB : MF) {
-    // FIXME: We retain the entry block and conservatively keep all landing pad
-    // blocks as part of the original function. Once D73739 is submitted, we can
-    // improve the handling of ehpads.
-    if ((MBB.pred_empty() || MBB.isEHPad()))
+    if (MBB.isEntryBlock())
       continue;
-    if (isColdBlock(MBB, MBFI, PSI))
+
+    if (MBB.isEHPad())
+      LandingPads.push_back(&MBB);
+    else if (isColdBlock(MBB, MBFI, PSI))
       MBB.setSectionID(MBBSectionID::ColdSectionID);
   }
 
+  // We only split out eh pads if all of them are cold.
+  bool HasHotLandingPads = false;
+  for (const MachineBasicBlock *LP : LandingPads) {
+    if (!isColdBlock(*LP, MBFI, PSI))
+      HasHotLandingPads = true;
+  }
+  if (!HasHotLandingPads) {
+    for (MachineBasicBlock *LP : LandingPads)
+      LP->setSectionID(MBBSectionID::ColdSectionID);
+  }
+
   auto Comparator = [](const MachineBasicBlock &X, const MachineBasicBlock &Y) {
     return X.getSectionID().Type < Y.getSectionID().Type;
   };

diff  --git a/llvm/test/CodeGen/X86/machine-function-splitter.ll b/llvm/test/CodeGen/X86/machine-function-splitter.ll
index 67edd7e17933..b4721ace2307 100644
--- a/llvm/test/CodeGen/X86/machine-function-splitter.ll
+++ b/llvm/test/CodeGen/X86/machine-function-splitter.ll
@@ -150,12 +150,12 @@ define void @foo6(i1 zeroext %0) nounwind section "nosplit" !prof !14 {
 }
 
 define i32 @foo7(i1 zeroext %0) personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) !prof !14 {
-;; Check that cold ehpads are not split out.
+;; Check that a single cold ehpad is split out.
 ; MFS-DEFAULTS-LABEL: foo7
 ; MFS-DEFAULTS:       .section        .text.split.foo7,"ax", at progbits
 ; MFS-DEFAULTS-NEXT:  foo7.cold:
-; MFS-DEFAULTS-NOT:   callq   _Unwind_Resume
 ; MFS-DEFAULTS:       callq   baz
+; MFS-DEFAULTS:       callq   _Unwind_Resume at PLT
 entry:
   invoke void @_Z1fv()
           to label %try.cont unwind label %lpad
@@ -182,6 +182,47 @@ try.cont:
   ret i32 %7
 }
 
+define i32 @foo8(i1 zeroext %0) personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) !prof !14 {
+;; Check that all ehpads are treated as hot if one of them is hot.
+; MFS-DEFAULTS-LABEL: foo8
+; MFS-DEFAULTS:       callq   _Unwind_Resume at PLT
+; MFS-DEFAULTS:       callq   _Unwind_Resume at PLT
+; MFS-DEFAULTS:       .section        .text.split.foo8,"ax", at progbits
+; MFS-DEFAULTS-NEXT:  foo8.cold:
+; MFS-DEFAULTS:       callq   baz
+entry:
+  invoke void @_Z1fv()
+          to label %try.cont unwind label %lpad1
+
+lpad1:
+  %1 = landingpad { i8*, i32 }
+          cleanup
+          catch i8* bitcast (i8** @_ZTIi to i8*)
+  resume { i8*, i32 } %1
+
+try.cont:
+  br i1 %0, label %hot, label %cold, !prof !17
+
+hot:
+  %2 = call i32 @bar()
+  invoke void @_Z1fv()
+          to label %exit unwind label %lpad2, !prof !21
+
+lpad2:
+  %3 = landingpad { i8*, i32 }
+          cleanup
+          catch i8* bitcast (i8** @_ZTIi to i8*)
+  resume { i8*, i32 } %3
+
+cold:
+  %4 = call i32 @baz()
+  br label %exit
+
+exit:
+  %5 = tail call i32 @qux()
+  ret i32 %5
+}
+
 declare i32 @bar()
 declare i32 @baz()
 declare i32 @bam()