[PATCH] D91756: [CSSPGO] Pseudo probes for function calls.

[Hongtao Yu via Phabricator via cfe-commits]

hoy added inline comments.


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Comment at: llvm/lib/CodeGen/PseudoProbeInserter.cpp:50
+    for (MachineBasicBlock &MBB : MF) {
+      MachineInstr *FirstInstr = nullptr;
+      for (MachineInstr &MI : MBB) {
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wmi wrote:
> What is the usage of FirstInstr?
Good patch. It's not used in this change. It's used in an upcoming change where a probe probing an empty block will be marked specially for better counts inference. 


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Comment at: llvm/lib/CodeGen/PseudoProbeInserter.cpp:54
+          FirstInstr = &MI;
+        if (MI.isCall()) {
+          if (DILocation *DL = MI.getDebugLoc()) {
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wmi wrote:
> Will tailcall or other optimizations convert call into something else before PseudoProbeInserter pass?
Good point. A tailcall instruction (a special MIR jump instruction like `TAILJMPd`) is considered as  a call and can be identified as  `MI.isCall() && MI.isTerminator() && MI.isReturn()`. Tail calls are bad to the frame-pointer-based virtual unwinding since they may cause missing frames. An upcoming change will mark them specially and a tail call tracker in `llvm-profgen` will try to track the missing frames based on some static analyses.


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Comment at: llvm/lib/Transforms/IPO/SampleProfileProbe.cpp:136-138
+  for (auto &I : CallProbeIds) {
+    auto Call = I.first;
+    uint32_t Index = I.second;
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wmi wrote:
> for (auto &[Call, Index] : CallProbeIds) {
Thanks for the suggestion. This is a C++17 usage and may cause a warning with the current build setup which defaults to C++14  (due to -Wc++17)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D91756/new/

https://reviews.llvm.org/D91756