StackProtector: ignore debug insts when splitting blocks.

[t.p.northover]

Summary

When deciding where to split a block to insert stack guard checks, we should move past any debug instructions we see that might (e.g.) be separating a tail call from its frame wrangling.

[aschwaighofer]

lgtm

[t.p.northover]

Thanks, committed:

To github.com:llvm/llvm-project.git

5d639034652d..7605ca85f1a8  main -> main

llvm/lib/CodeGen/CodeGenCommonISel.cpp

 llvm::findSplitPointForStackProtector(MachineBasicBlock *BB,
                                       const TargetInstrInfo &TII) {
   MachineBasicBlock::iterator SplitPoint = BB->getFirstTerminator();
   if (SplitPoint == BB->begin())
     return SplitPoint;
 
   MachineBasicBlock::iterator Start = BB->begin();
   MachineBasicBlock::iterator Previous = SplitPoint;
+  do {
     --Previous;
+  } while (Previous->isDebugInstr());
 
   if (TII.isTailCall(*SplitPoint) &&
       Previous->getOpcode() == TII.getCallFrameDestroyOpcode()) {
     // Call frames cannot be nested, so if this frame is describing the tail
     // call itself, then we must insert before the sequence even starts. For
     // example:
     //     <split point>
     //     ADJCALLSTACKDOWN ...
     //     <Moves>
     //     ADJCALLSTACKUP ...
     //     TAILJMP somewhere
     // On the other hand, it could be an unrelated call in which case this tail
-    // call has to register moves of its own and should be the split point. For
+    // call has no register moves of its own and should be the split point. For
     // example:
     //     ADJCALLSTACKDOWN
     //     CALL something_else
     //     ADJCALLSTACKUP
     //     <split point>
     //     TAILJMP somewhere
     do {
       --Previous;

llvm/test/CodeGen/AArch64/tailcall-ssp-split-debug.ll

+; RUN: llc -mtriple=arm64-apple-ios %s -o - | FileCheck %s
+
+define swifttailcc void @foo(i8* %call) ssp {
+; CHECK-LABEL: foo:
+  %var = alloca [28 x i8], align 16
+  br i1 undef, label %if.then, label %if.end
+
+if.then:
+  ret void
+
+if.end:
+  ; CHECK: mov x[[NULL:[0-9]+]], xzr
+  ; CHECK: ldr [[FPTR:x[0-9]+]], [x[[NULL]]]
+  ; CHECK: br [[FPTR]]
+  call void @llvm.dbg.value(metadata i8* %call, metadata !19, metadata !DIExpression()), !dbg !21
+  %fptr = load void (i8*)*, void (i8*)** null, align 8
+  musttail call swifttailcc void %fptr(i8* null)
+  ret void
+}
+
+declare i8* @pthread_getspecific()
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+!llvm.module.flags = !{!2}
+!llvm.dbg.cu = !{!11}
+
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = distinct !DICompileUnit(language: DW_LANG_C99, file: !12, producer: "Apple clang version 13.1.6 (clang-1316.0.17.4)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !13, splitDebugInlining: false, nameTableKind: None, sysroot: "/Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk", sdk: "MacOSX12.3.sdk")
+!12 = !DIFile(filename: "tmp.c", directory: "/Users/tim/llvm-internal/llvm-project/build")
+!13 = !{}
+!14 = !{!"Apple clang version 13.1.6 (clang-1316.0.17.4)"}
+!15 = distinct !DISubprogram(name: "bar", scope: !12, file: !12, line: 3, type: !16, scopeLine: 3, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !11, retainedNodes: !18)
+!16 = !DISubroutineType(types: !17)
+!17 = !{null}
+!18 = !{!19}
+!19 = !DILocalVariable(name: "var", scope: !15, file: !12, line: 4, type: !20)
+!20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!21 = !DILocation(line: 0, scope: !15)