[EHStreamer] Ensure CallSiteEntry::{BeginLabel,EndLabel} are non-null. NFC

[MaskRay]

Summary

... to simplify the code a bit.

[MaskRay]

Simplify

[rahmanl]

llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp

273

I am not sure if this is correct. When reaching the end of one MBB, we must be able to use the end label of that MBB and the begin label here.

Instead, why don't you change the definition of LastLabel to
MCSymbol *LastLabel = Asm->getFunctionBegin();

Then you don't need to change this LastLabel argument here and the line below.

517

Good cleanup!

[MaskRay]

Instead, why don't you change the definition of LastLabel to
MCSymbol *LastLabel = Asm->getFunctionBegin();

Then you don't need to change this LastLabel argument here and the line below.

Thanks, good suggestion! Adopted.

[rahmanl]

LGTM.
Please just keep in mind that call-site-table is not rigorously tested in LLVM.

[MaskRay]

In D87999#2285800, @rahmanl wrote:

LGTM.
Please just keep in mind that call-site-table is not rigorously tested in LLVM.

*Nod* I'll commit this tomorrow so that you can rebase :)

[MaskRay]

@rahmanl The patch will look better if you can give a more formal approval :)

llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp

for (const auto &MI : MBB) {
       assert(BeginLabel == LandingPad->BeginLabels[P.RangeIndex] &&
              "Inconsistent landing pad map!");
 
       // For Dwarf exception handling (SjLj handling doesn't use this). If some
       // instruction between the previous try-range and this one may throw,
       // create a call-site entry with no landing pad for the region between the
       // try-ranges.
       if (SawPotentiallyThrowing && Asm->MAI->usesCFIForEH()) {
-        CallSiteEntry Site = { LastLabel, BeginLabel, nullptr, 0 };
+        CallSiteEntry Site = {Asm->getFunctionBegin(), BeginLabel, nullptr, 0};
         CallSites.push_back(Site);
         PreviousIsInvoke = false;
       }
 
       LastLabel = LandingPad->EndLabels[P.RangeIndex];
       assert(BeginLabel && LastLabel && "Invalid landing pad!");
 
       if (!LandingPad->LandingPadLabel) {
         // Create a gap.
for (const auto &MI : MBB) {
       }
     }
   }
 
   // If some instruction between the previous try-range and the end of the
   // function may throw, create a call-site entry with no landing pad for the
   // region following the try-range.
   if (SawPotentiallyThrowing && !IsSJLJ) {
-    CallSiteEntry Site = { LastLabel, nullptr, nullptr, 0 };
+    if (LastLabel == nullptr)
+      LastLabel = Asm->getFunctionBegin();
+    CallSiteEntry Site = {LastLabel, Asm->getFunctionEnd(), nullptr, 0};
     CallSites.push_back(Site);
   }
 }
 
 /// Emit landing pads and actions.
 ///
 /// The general organization of the table is complex, but the basic concepts are
 /// easy.  First there is a header which describes the location and organization
if (IsSJLJ || IsWasm) {
 
     unsigned Entry = 0;
     for (SmallVectorImpl<CallSiteEntry>::const_iterator
          I = CallSites.begin(), E = CallSites.end(); I != E; ++I) {
       const CallSiteEntry &S = *I;
 
       MCSymbol *EHFuncBeginSym = Asm->getFunctionBegin();
 
-      MCSymbol *BeginLabel = S.BeginLabel;
-      if (!BeginLabel)
-        BeginLabel = EHFuncBeginSym;
-      MCSymbol *EndLabel = S.EndLabel;
-      if (!EndLabel)
-        EndLabel = Asm->getFunctionEnd();
-
       // Offset of the call site relative to the start of the procedure.
       if (VerboseAsm)
         Asm->OutStreamer->AddComment(">> Call Site " + Twine(++Entry) + " <<");
-      Asm->emitCallSiteOffset(BeginLabel, EHFuncBeginSym, CallSiteEncoding);
+      Asm->emitCallSiteOffset(S.BeginLabel, EHFuncBeginSym, CallSiteEncoding);
       if (VerboseAsm)
         Asm->OutStreamer->AddComment(Twine("  Call between ") +
-                                     BeginLabel->getName() + " and " +
-                                     EndLabel->getName());
-      Asm->emitCallSiteOffset(EndLabel, BeginLabel, CallSiteEncoding);
+                                     S.BeginLabel->getName() + " and " +
+                                     S.EndLabel->getName());
+      Asm->emitCallSiteOffset(S.EndLabel, S.BeginLabel, CallSiteEncoding);
 
       // Offset of the landing pad relative to the start of the procedure.
       if (!S.LPad) {
         if (VerboseAsm)
           Asm->OutStreamer->AddComment("    has no landing pad");
         Asm->emitCallSiteValue(0, CallSiteEncoding);
       } else {
         if (VerboseAsm)