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

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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D131626

Author: rlavaee

llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp

@@ -663,9 +663,10 @@ MCSymbol *EHStreamer::emitExceptionTable() {
       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

llvm/test/CodeGen/X86/gcc_except_table_bb_sections_nolpads.ll

@@ -0,0 +1,44 @@
+;; 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 < %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-NEXT:   .byte	255                             # @TType Encoding = omit
+; CHECK-NEXT:   .byte	1                               # Call site Encoding = uleb128
+; CHECK-NEXT:   .uleb128 .Laction_table_base0-.Lcst_begin0
+; CHECK-NEXT: .Lcst_begin0:
+; CHECK-NEXT: .Lexception1:
+; CHECK-NEXT:   .byte	255                             # @LPStart Encoding = omit
+; CHECK-NEXT:   .byte	255                             # @TType Encoding = omit
+; CHECK-NEXT:   .byte	1                               # Call site Encoding = uleb128
+; CHECK-NEXT:   .uleb128 .Laction_table_base0-.Lcst_begin1
+; CHECK-NEXT: .Lcst_begin1:
+; CHECK-NEXT: .Lexception2:
+; CHECK-NEXT:   .byte	255                             # @LPStart Encoding = omit
+; CHECK-NEXT:   .byte	255                             # @TType Encoding = omit
+; CHECK-NEXT:   .byte	1                               # Call site Encoding = uleb128
+; CHECK-NEXT:   .uleb128 .Laction_table_base0-.Lcst_begin2
+; CHECK-NEXT: .Lcst_begin2:
+; CHECK-NEXT:   .uleb128 foo.__part.2-foo.__part.2      # >> Call Site 1 <<
+; CHECK-NEXT:   .uleb128 .LBB_END0_2-foo.__part.2       #   Call between foo.__part.2 and .LBB_END0_2
+; CHECK-NEXT:   .byte	0                               #     has no landing pad
+; CHECK-NEXT:   .byte	0                               #   On action: cleanup
+; CHECK-NEXT: .Laction_table_base0: