[llvm] r310985 - Merge debug info when hoist then-else code to if.

Sat Aug 26 10:05:59 PDT 2017

> On Aug 25, 2017, at 5:09 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Fri, Aug 25, 2017 at 4:44 PM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> On Aug 25, 2017, at 3:15 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> Adrian - what were we doing about cases where merged locations were on call instructions? I think this case is one of those, where if the instruction is a call, this may drop the location from the call, then inline through the call creating corrupted debug info.
> 
> I guess that in this case the only sane thing we can do is create a new artificial line 0 location in one of the two calls' scope. It's not pretty, but that;s the best I can come up with.
> 
> A few options:
> 
> Further up in the same file, the code avoids merging if the instruction is a call (originally r380995) SimplifyCFG.cpp:1282.
> 
> So that's one option - do nothing on calls. But it's probably not ideal/wrong (since it's not either location)
> 
> Beyond that we get to the zero location - but in what scope?

I don't have a good answer for that. It looks like whatever we do will screw up the backtraces in at least some cases.

> The root scope of the function? (what if these two calls are both within a single inlined call? That'll punch a hole in the range which isn't entirely accurate) The nearest common parent scope to the two instructions (assuming they both have debug locations? If one doesn't have a location, could we reliably use the other location? Probably not - so if one doesn't have a location we should use the zero location at the root of the current function for sure)
> Worth finding the common parent scope?

That sounds like a workable compromise.

> Should this functionality be built into getMergedLocation?

I think so — we could add an isCall bool argument or — probably even better — pass in the instruction the location will be attached to.

-- adrian

>  
> 
> -- adrian
> 
>> 
>> Here's a test case that seems to exercise/assert on that (but it asserts relatively late - would be handy to have a way to run the verifier between each optimization - maybe there is such an option?):
>> 
>> struct string {
>>   ~string();
>> };
>> void f2();
>> void f1(int) { f2(); }
>> void run(int c) {
>>   string body;
>>   while (true) {
>>     if (c)
>>       f1(c);
>>     else
>>       f1(c);
>>   }
>> }
>> 
>> On Tue, Aug 15, 2017 at 6:56 PM Dehao Chen via llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
>> Author: dehao
>> Date: Tue Aug 15 18:55:26 2017
>> New Revision: 310985
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=310985&view=rev <http://llvm.org/viewvc/llvm-project?rev=310985&view=rev>
>> Log:
>> Merge debug info when hoist then-else code to if.
>> 
>> Summary: When we move then-else code to if, we need to merge its debug info, otherwise the hoisted instruction may have inaccurate debug info attached.
>> 
>> Reviewers: aprantl, probinson, dblaikie, echristo, loladiro
>> 
>> Reviewed By: aprantl
>> 
>> Subscribers: sanjoy, llvm-commits
>> 
>> Differential Revision: https://reviews.llvm.org/D36778 <https://reviews.llvm.org/D36778>
>> 
>> Added:
>>     llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll
>> Modified:
>>     llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
>> 
>> Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=310985&r1=310984&r2=310985&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=310985&r1=310984&r2=310985&view=diff>
>> ==============================================================================
>> --- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
>> +++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Tue Aug 15 18:55:26 2017
>> @@ -1336,6 +1336,8 @@ HoistTerminator:
>>      I2->replaceAllUsesWith(NT);
>>      NT->takeName(I1);
>>    }
>> +  NT->setDebugLoc(DILocation::getMergedLocation(
>> +      I1->getDebugLoc(), I2->getDebugLoc()));
>> 
>>    IRBuilder<NoFolder> Builder(NT);
>>    // Hoisting one of the terminators from our successor is a great thing.
>> 
>> Added: llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll?rev=310985&view=auto <http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll?rev=310985&view=auto>
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll (added)
>> +++ llvm/trunk/test/Transforms/SimplifyCFG/min_dbginfo.ll Tue Aug 15 18:55:26 2017
>> @@ -0,0 +1,39 @@
>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s
>> +
>> +; Checks if the debug info is removed for the "select" instruction.
>> +; CHECK: cmp {{.*}} !dbg
>> +; CHECK-NOT: select {{.*}} !dbg
>> +define i32 @min(i32 %a, i32 %b) {
>> +entry:
>> +  %cmp = icmp slt i32 %a, %b, !dbg !9
>> +  br i1 %cmp, label %if.then, label %if.else, !dbg !10
>> +
>> +if.then:
>> +  br label %return, !dbg !11
>> +
>> +if.else:
>> +  br label %return, !dbg !12
>> +
>> +return:
>> +  %retval.0 = phi i32 [ %a, %if.then ], [ %b, %if.else ]
>> +  ret i32 %retval.0, !dbg !13
>> +}
>> +
>> +!llvm.dbg.cu <http://llvm.dbg.cu/> = !{!0}
>> +!llvm.module.flags = !{!3, !4, !5}
>> +!llvm.ident = !{!6}
>> +
>> +!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 6.0.0 (trunk 310792)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
>> +!1 = !DIFile(filename: "min.cc <http://min.cc/>", directory: "/")
>> +!2 = !{}
>> +!3 = !{i32 2, !"Dwarf Version", i32 4}
>> +!4 = !{i32 2, !"Debug Info Version", i32 3}
>> +!5 = !{i32 1, !"wchar_size", i32 4}
>> +!6 = !{!"clang version 6.0.0 (trunk 310792)"}
>> +!7 = distinct !DISubprogram(name: "min", scope: !1, file: !1, line: 3, type: !8, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
>> +!8 = !DISubroutineType(types: !2)
>> +!9 = !DILocation(line: 4, column: 8, scope: !7)
>> +!10 = !DILocation(line: 4, column: 6, scope: !7)
>> +!11 = !DILocation(line: 5, column: 3, scope: !7)
>> +!12 = !DILocation(line: 7, column: 3, scope: !7)
>> +!13 = !DILocation(line: 9, column: 1, scope: !7)
>> 
>> 
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