[LoopStrengthReduce] Ensure that debug intrinsics do not affect LSR's output

[StephenTozer]

Summary

Fixes bug 50427: https://bugs.llvm.org/show_bug.cgi?id=50427

During Loop Strength Reduce, if the terminating condition for the loop is not immediately adjacent to the terminating branch and it has more than one use, a clone of the condition will be created just before the terminating branch and will be used as the branch condition. Currently, whether the instructions are "immediately adjacent" is determined by checking whether the next instruction after the condition is the terminating branch; this is incorrect however, as the presence of a debug intrinsic between the two will result in a change to the output. This is fixed by using getNextNonDebugInstruction() instead.

[dblaikie]

Any chance of reducing the test case further?

[djtodoro]

lgtm (some nits/advices on how-to-reduce-test-case inline)

llvm/test/Transforms/LoopStrengthReduce/X86/lsr-cond-dbg.ll

109

!tbaa not needed I guess?

112

All these DILocations with the same scope could be represented as one DILocation metadata, so we can get rid of the rest.

[StephenTozer]

In D103033#2778091, @dblaikie wrote:

Any chance of reducing the test case further?

I can try - it's already been put through creduce and bugpoint, but there may be some value to be gained from manual editing the file.

[StephenTozer]

Reduced test file by ~25%.

[StephenTozer]

Ping.

[djtodoro]

lgtm (+ nit)

llvm/test/Transforms/LoopStrengthReduce/X86/lsr-cond-dbg.ll

112

'No newline at end of file"

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

if (LatchBlock != ExitingBlock)
         }
 
     LLVM_DEBUG(dbgs() << "  Change loop exiting icmp to use postinc iv: "
                       << *Cond << '\n');
 
     // It's possible for the setcc instruction to be anywhere in the loop, and
     // possible for it to have multiple users.  If it is not immediately before
     // the exiting block branch, move it.
-    if (&*++BasicBlock::iterator(Cond) != TermBr) {
+    if (Cond->getNextNonDebugInstruction() != TermBr) {
       if (Cond->hasOneUse()) {
         Cond->moveBefore(TermBr);
       } else {
         // Clone the terminating condition and insert into the loopend.
         ICmpInst *OldCond = Cond;
         Cond = cast<ICmpInst>(Cond->clone());
         Cond->setName(L->getHeader()->getName() + ".termcond");
         ExitingBlock->getInstList().insert(TermBr->getIterator(), Cond);

llvm/test/Transforms/LoopStrengthReduce/X86/lsr-cond-dbg.ll

+; RUN: opt -S -loop-reduce < %s | FileCheck %s
+
+; During Loop Strength Reduce, if the terminating condition for the loop is not
+; immediately adjacent to the terminating branch and it has more than one use,
+; a clone of the condition will be created just before the terminating branch
+; and will be used as the branch condition.
+; The purpose of this test is to check that the presence of a debug intrinsic
+; between the condition and branch does not trigger this behaviour, as this
+; would cause debug info to affect CodeGen.
+
+; CHECK-LABEL: i:
+; CHECK-NOT: icmp
+; CHECK: [[COND:%.*]] = icmp eq i8
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i1 [[COND]]
+; CHECK-NEXT: br i1 [[COND]]
+
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@b = dso_local local_unnamed_addr global i8 0, align 1, !dbg !0
+@a = dso_local local_unnamed_addr global i32 0, align 4, !dbg !6
+@c = dso_local local_unnamed_addr global i8 0, align 1, !dbg !9
+define dso_local signext i16 @d() local_unnamed_addr !dbg !17 {
+entry:
+  %0 = load i32, i32* @a, align 4
+  %tobool2.not = icmp eq i32 %0, 0
+  %1 = load i8, i8* @b, align 1, !dbg !24
+  %cmp.not13 = icmp eq i8 %1, 7, !dbg !24
+  br i1 %cmp.not13, label %cleanup, label %if.end.preheader, !dbg !24
+
+if.end.preheader:                                 ; preds = %entry
+  br label %if.end, !dbg !24
+
+if.end:                                           ; preds = %if.end.preheader, %i
+  %2 = phi i8 [ %add, %i ], [ %1, %if.end.preheader ]
+  br i1 %tobool2.not, label %i, label %if.end4, !dbg !24
+
+if.end4:                                          ; preds = %if.end
+  %conv5 = trunc i32 %0 to i16, !dbg !24
+  br label %cleanup, !dbg !24
+
+i:                                                ; preds = %if.end
+  %add = add i8 %2, 1, !dbg !24
+  store i8 %add, i8* @b, align 1, !dbg !24
+  %cmp.not = icmp eq i8 %add, 7, !dbg !24
+  call void @llvm.dbg.value(metadata i1 %cmp.not, metadata !23, metadata !DIExpression(DW_OP_LLVM_convert, 1, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !24
+  br i1 %cmp.not, label %cleanup.loopexit, label %if.end, !dbg !24
+
+cleanup.loopexit:                                 ; preds = %i
+  br label %cleanup
+
+cleanup:                                          ; preds = %cleanup.loopexit, %if.end4, %entry
+  %cmp.not12 = phi i1 [ %cmp.not13, %if.end4 ], [ %cmp.not13, %entry ], [ %cmp.not, %cleanup.loopexit ]
+  %retval.0 = phi i16 [ %conv5, %if.end4 ], [ undef, %entry ], [ undef, %cleanup.loopexit ]
+  %3 = load i8, i8* @c, align 1
+  %conv8 = sext i8 %3 to i16
+  %retval.1 = select i1 %cmp.not12, i16 %conv8, i16 %retval.0
+  ret i16 %retval.1, !dbg !24
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!12, !13, !14, !15}
+!llvm.ident = !{!16}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 2, type: !11, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 13.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "lsr-cond-dbg.c", directory: "/")
+!4 = !{}
+!5 = !{!6, !0, !9}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!9 = !DIGlobalVariableExpression(var: !10, expr: !DIExpression())
+!10 = distinct !DIGlobalVariable(name: "c", scope: !2, file: !3, line: 2, type: !11, isLocal: false, isDefinition: true)
+!11 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!12 = !{i32 7, !"Dwarf Version", i32 4}
+!13 = !{i32 2, !"Debug Info Version", i32 3}
+!14 = !{i32 1, !"wchar_size", i32 4}
+!15 = !{i32 7, !"uwtable", i32 1}
+!16 = !{!"clang version 13.0.0"}
+!17 = distinct !DISubprogram(name: "d", scope: !3, file: !3, line: 3, type: !18, scopeLine: 3, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !21)
+!18 = !DISubroutineType(types: !19)
+!19 = !{!20}
+!20 = !DIBasicType(name: "short", size: 16, encoding: DW_ATE_signed)
+!21 = !{!23}
+!22 = distinct !DILexicalBlock(scope: !17, file: !3, line: 4, column: 3)
+!23 = !DILocalVariable(name: "g", scope: !22, file: !3, line: 7, type: !8)
+!24 = !DILocation(line: 5, column: 9, scope: !22)
+