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<p class="MsoNormal"><span lang="EN-US">The Loop Strength Reduction pass appears to break debug information even for<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">the most basic input. I believe this is a well known issue already (see<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">https://bugs.llvm.org/show_bug.cgi?id=38815) but I also believe that it deserve<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">some extra attention.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Consider the following input compiled with 'clang -g -O3 foo.c -mllvm -print-after-all'<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">---<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">void foo(unsigned char *p) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">#pragma clang loop unroll(disable)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> for (unsigned char i = 0; i < 32; i++) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> p += 3;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> *p = i;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">---<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">As can be seen below in the pre-LSR dump the pointer 'p' is desribed by<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">dbg.value both before and after it is incremented. In the post-LSR setting<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">these dbg.value are now referring to undef so the debug-info for 'p' is<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">effectively lost.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">It is interesting to note that after LSR one dbg.value has its DIExpression<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">adjusted to add the value three but it is still referring to undef. As I<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">understand it this is the work of salvageDebugInfo. When LSR has created the<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">new induction variable expression and starts deleting the old one salvaging<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">will kick in. When %add.ptr is deleted it will adjust the dbg.value use of<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">that to use the value %p.addr.05, and by doing so it must accordingly adjust the<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">DIExpression. This all works fine until the phi-node is also deleted at which<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">point there is not much that can be done with the current scheme.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">One idea for how to address this would be that, since LSR is a SCEV based<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">optimization, one could perform additional debug salvaging by comparing SCEV<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">expressions for the new and old PHI-node and then adjusting DIExpressions if<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">they match with an offset.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Any thoughts on that?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">-Markus<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">---<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">*** IR Dump After Canonicalize Freeze Instructions in Loops ***<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Preheader:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">entry:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* %p, metadata <0x111daf20>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 0, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> br label %for.body, !dbg !DILocation(line: 3, column: 3, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Loop:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">for.body: ; preds = %entry, %for.body<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %i.06 = phi i8 [ 0, %entry ], [ %inc, %for.body ]<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %p.addr.05 = phi i8* [ %p, %entry ], [ %add.ptr, %for.body ]<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 %i.06, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* %p.addr.05, metadata <0x111daf20>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %add.ptr = getelementptr inbounds i8, i8* %p.addr.05, i64 3, !dbg !DILocation(line: 4, column: 7, scope: <0x111ddc90>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* %add.ptr, metadata <0x111daf20>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> store i8 %i.06, i8* %add.ptr, align 1, !dbg !DILocation(line: 5, column: 8, scope: <0x111ddc90>), !tbaa <0x111dd288><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %inc = add nuw nsw i8 %i.06, 1, !dbg !DILocation(line: 3, column: 38, scope: <0x111dd510>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 %inc, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %exitcond.not = icmp eq i8 %inc, 32, !dbg !DILocation(line: 3, column: 31, scope: <0x111dd510>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> br i1 %exitcond.not, label %for.cond.cleanup, label %for.body, !dbg !DILocation(line: 3, column: 3, scope: <0x111dc650>), !llvm.loop <0x111de7b0><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Exit blocks<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">for.cond.cleanup: ; preds = %for.body<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> ret void, !dbg !DILocation(line: 7, column: 1, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">*** IR Dump After Loop Strength Reduction ***<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Preheader:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">entry:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* %p, metadata <0x111daf20>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 0, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %scevgep = getelementptr i8, i8* %p, i64 3, !dbg !DILocation(line: 3, column: 3, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> br label %for.body, !dbg !DILocation(line: 3, column: 3, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Loop:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">for.body: ; preds = %entry, %for.body<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %lsr.iv = phi i8* [ %scevgep, %entry ], [ %scevgep7, %for.body ]<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %i.06 = phi i8 [ 0, %entry ], [ %inc, %for.body ]<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 %i.06, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* undef, metadata <0x111daf20>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8* undef, metadata <0x111daf20>, metadata !DIExpression(DW_OP_plus_uconst, 3, DW_OP_stack_value)), !dbg !DILocation(line: 0, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> store i8 %i.06, i8* %lsr.iv, align 1, !dbg !DILocation(line: 5, column: 8, scope: <0x111ddc90>), !tbaa <0x111dd288><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %inc = add nuw nsw i8 %i.06, 1, !dbg !DILocation(line: 3, column: 38, scope: <0x111dd510>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> call void @llvm.dbg.value(metadata i8 %inc, metadata <0x111dcb00>, metadata !DIExpression()), !dbg !DILocation(line: 0, scope: <0x111dc650>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %scevgep7 = getelementptr i8, i8* %lsr.iv, i64 3, !dbg !DILocation(line: 3, column: 31, scope: <0x111dd510>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %exitcond.not = icmp eq i8 %inc, 32, !dbg !DILocation(line: 3, column: 31, scope: <0x111dd510>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> br i1 %exitcond.not, label %for.cond.cleanup, label %for.body, !dbg !DILocation(line: 3, column: 3, scope: <0x111dc650>), !llvm.loop <0x111de7b0><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">; Exit blocks<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">for.cond.cleanup: ; preds = %for.body<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> ret void, !dbg !DILocation(line: 7, column: 1, scope: <0x111d4030>)<o:p></o:p></span></p>
<p class="MsoNormal">---<o:p></o:p></p>
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