[EHStreamer] Omit @LPStart when function has no landing pads

[rahmanl]

Summary

When no landing pads exist for a function, @LPStart is undefined and must be omitted.

EH table is generally not emitted for functions without landing pads, except when the personality function is uknown (!isNoOpWithoutInvoke(classifyEHPersonality(Per))). In that case, we must omit @LPStart even when machine function splitting is enabled.

[MaskRay]

Test case?

[MaskRay]

The subject can remove the ending period. Either [AsmPrinter] or [EHStreamer] is a good tag.

[rahmanl]

Add test case.

[rahmanl]

Revised the comment.

[MaskRay]

llvm/test/CodeGen/X86/gcc_except_table_bb_sections_nolpads.ll

4

-code-model=small can be removed.

27

Consider checking the full content, i.e. .uleb128 .Laction_table_base0-.Lcst_begin0 and .uleb128 foo.__part.2-foo.__part.2 can be interesting as well.

[MaskRay]

Perhaps mention in the description that this is related to !isNoOpWithoutInvoke(classifyEHPersonality(Per)) in DwarfCFIException.cpp

[rahmanl]

Include full content of EH table in the test.

[rahmanl]

Thanks for the review @MaskRay

[rahmanl]

rebase.

llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp

for (const CallSiteRange &CSRange : CallSiteRanges) {
       if (CSRange.CallSiteBeginIdx != 0) {
         // Align the call-site range for all ranges except the first. The
         // first range is already aligned due to the exception table alignment.
         Asm->emitAlignment(Align(4));
       }
       Asm->OutStreamer->emitLabel(CSRange.ExceptionLabel);
 
       // Emit the LSDA header.
-      // If only one call-site range exists, LPStart is omitted as it is the
-      // same as the function entry.
-      if (CallSiteRanges.size() == 1) {
+      // LPStart is omitted if either we have a single call-site range (in which
+      // case the function entry is treated as @LPStart) or if this function has
+      // no landing pads (in which case @LPStart is undefined).
+      if (CallSiteRanges.size() == 1 || LandingPadRange == nullptr) {
         Asm->emitEncodingByte(dwarf::DW_EH_PE_omit, "@LPStart");
       } else if (!Asm->isPositionIndependent()) {
         // For more than one call-site ranges, LPStart must be explicitly
         // specified.
         // For non-PIC we can simply use the absolute value.
         Asm->emitEncodingByte(dwarf::DW_EH_PE_absptr, "@LPStart");
         Asm->OutStreamer->emitSymbolValue(LandingPadRange->FragmentBeginLabel,
                                           Asm->MAI->getCodePointerSize());

llvm/test/CodeGen/X86/gcc_except_table_bb_sections_nolpads.ll

+;; Verify that @LPStart is omitted when there are no landing pads. This test
+;; uses an unkown personality to force emitting the exception table.
+
+; RUN: llc -basic-block-sections=all -mtriple=x86_64 -code-model=small < %s | FileCheck %s
+
+declare void @throwit()
+declare i32 @__unknown_ehpersonality(...)
+
+define void @foo(i1 %cond) uwtable personality ptr @__unknown_ehpersonality {
+entry:
+  br i1 %cond, label %cond.true, label %cond.false
+
+cond.true:                                        ; preds = %entry
+  call void @throwit()
+  unreachable
+
+cond.false:                                         ; preds = %entry
+  ret void
+}
+
+; CHECK:      GCC_except_table0:
+; CHECK-NEXT: .Lexception0:
+; CHECK-NEXT:    .byte   255                             # @LPStart Encoding = omit
+; CHECK:      .Lexception1:
+; CHECK-NEXT:    .byte   255                             # @LPStart Encoding = omit
+; CHECK:      .Lexception2:
+; CHECK-NEXT:    .byte   255                             # @LPStart Encoding = omit