[RemoveDIs] Replicate dbg intrinsic movement pattern in SelectOptimize (#81737)

Fix crash mentioned in comments on
d759618.

The assertion being hit was complaining that we had dangling DPValues;
the DPValues attached to the terminator of StartBlock become dangling
after the terminator is erased, and they're never "flushed" back onto
the new terminator once it's added. Doing that makes the crash go away,
but doesn't replicate existing dbg.* behaviour. See the comment in the
patch.

This change both fixes the crash (because there are now no DPValues left
on the terminator to dangle) and replicates existing behaviour (moves
those DPValues down to the new block).

Author: OCHyams

llvm/lib/CodeGen/SelectOptimize.cpp

@@ -621,6 +621,12 @@ void SelectOptimizeImpl::convertProfitableSIGroups(SelectGroups &ProfSIGroups) {
     SelectLike LastSI = ASI.back();
     BasicBlock *StartBlock = SI.getI()->getParent();
     BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(LastSI.getI()));
+    // With RemoveDIs turned off, SplitPt can be a dbg.* intrinsic. With
+    // RemoveDIs turned on, SplitPt would instead point to the next
+    // instruction. To match existing dbg.* intrinsic behaviour with RemoveDIs,
+    // tell splitBasicBlock that we want to include any DPValues attached to
+    // SplitPt in the splice.
+    SplitPt.setHeadBit(true);
     BasicBlock *EndBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");
     BFI->setBlockFreq(EndBlock, BFI->getBlockFreq(StartBlock));
     // Delete the unconditional branch that was just created by the split.

llvm/test/DebugInfo/AArch64/select-optimize-trailing-dbg-records.ll

@@ -0,0 +1,63 @@
+; RUN: opt %s -passes='require<profile-summary>,function(select-optimize)' -o - -S \
+; RUN: | FileCheck %s
+; RUN: opt %s --try-experimental-debuginfo-iterators -passes='require<profile-summary>,function(select-optimize)' -o - -S \
+; RUN: | FileCheck %s
+
+;; Check that the dbg.value is moved into the start of the end-block of the
+;; inserted if-block.
+
+; CHECK: select.end:
+; CHECK-NEXT: %[[PHI:.*]] = phi i32
+; CHECK-NEXT: dbg.value(metadata i32 %[[PHI]],
+
+source_filename = "test.ll"
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-unknown-fuchsia"
+
+%struct.hb_glyph_info_t = type { i32, i32, i32, %union._hb_var_int_t, %union._hb_var_int_t }
+%union._hb_var_int_t = type { i32 }
+
+define void @_Z22_hb_ot_shape_normalizePK18hb_ot_shape_plan_tP11hb_buffer_tP9hb_font_t() {
+entry:
+  br label %while.body193
+
+while.body193:                                    ; preds = %while.body193, %entry
+  %starter.0337 = phi i32 [ %spec.select322, %while.body193 ], [ 0, %entry ]
+  %idxprom207 = zext i32 %starter.0337 to i64
+  %arrayidx208 = getelementptr %struct.hb_glyph_info_t, ptr null, i64 %idxprom207
+  %0 = load i32, ptr %arrayidx208, align 4
+  %call247.val = load i16, ptr null, align 4
+  %cmp249327 = icmp ult i16 %call247.val, 0
+  %cmp249 = select i1 false, i1 false, i1 %cmp249327
+  %spec.select322 = select i1 %cmp249, i32 0, i32 %starter.0337
+  tail call void @llvm.dbg.value(metadata i32 %spec.select322, metadata !13, metadata !DIExpression()), !dbg !20
+  br label %while.body193
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!12}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !3, imports: !2, splitDebugInlining: false, nameTableKind: GNU)
+!1 = !DIFile(filename: "../../third_party/harfbuzz-ng/src/src/hb-ot-shape-normalize.cc", directory: ".")
+!2 = !{}
+!3 = !{!4, !9}
+!4 = !DIGlobalVariableExpression(var: !5, expr: !DIExpression())
+!5 = distinct !DIGlobalVariable(scope: null, file: !1, line: 383, type: !6, isLocal: true, isDefinition: true)
+!6 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, size: 112, elements: !2)
+!7 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !8)
+!8 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_unsigned_char)
+!9 = !DIGlobalVariableExpression(var: !10, expr: !DIExpression())
+!10 = distinct !DIGlobalVariable(scope: null, file: !1, line: 410, type: !11, isLocal: true, isDefinition: true)
+!11 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, size: 96, elements: !2)
+!12 = !{i32 2, !"Debug Info Version", i32 3}
+!13 = !DILocalVariable(name: "starter", scope: !14, file: !1, line: 441, type: !19)
+!14 = distinct !DILexicalBlock(scope: !15, file: !1, line: 435, column: 3)
+!15 = distinct !DILexicalBlock(scope: !16, file: !1, line: 431, column: 7)
+!16 = distinct !DISubprogram(name: "_hb_ot_shape_normalize", linkageName: "_Z22_hb_ot_shape_normalizePK18hb_ot_shape_plan_tP11hb_buffer_tP9hb_font_t", scope: !1, file: !1, line: 291, type: !17, scopeLine: 294, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!17 = distinct !DISubroutineType(types: !18)
+!18 = !{null}
+!19 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!20 = !DILocation(line: 0, scope: !14)