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-indvars miscompilation of Qt #1387

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asl opened this issue Nov 28, 2006 · 5 comments
Closed

-indvars miscompilation of Qt #1387

asl opened this issue Nov 28, 2006 · 5 comments
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@lattner
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bugzilla Issues migrated from bugzilla miscompilation

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@asl
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asl commented Nov 28, 2006

Bugzilla Link 1015
Resolution FIXED
Resolved on Feb 22, 2010 12:41
Version 1.5
OS All
Attachments Source file [qapp.ll], Opt output [qappl.out.ll]

Extended Description

Consider the input file qappl.ll. Look into bb101 and next BBs. This code
corresponds to the following code in cpp (this is extract from Qt):

for (i=0; (UINT)mouseTbl[i] != msg.message || !mouseTbl[i]; i += 3)
    ;
if (!mouseTbl[i])
    return false;
type = (QEvent::Type)mouseTbl[++i];
button = mouseTbl[++i];

qappl.out.ll is output from opt -indvars. We have:

    %tmp107 = seteq uint %tmp103, %tmp106           ; <bool> [#uses=1]
    br bool %tmp107, label %cond_next109, label %bb98

cond_next109: ; preds = %bb101
%exitcond = setne uint %indvar, 0 ; [#uses=1]
br bool %exitcond, label %bb98, label %bb114

%tmp103 corresponds to mouseTbl[i], %tmp106 - to msg.message. %indvar
corresponds to i/3.

Let us assume, that they equals, when i!=0. We have:

%tmp107 = true => branch to cond_next109 => %exitcond = true => branch to bb98

And we have infinite loop here :(
@asl
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asl commented Nov 28, 2006

assigned to @lattner

@lattner
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lattner commented Nov 28, 2006

Devang, can you take a look at this when you get a chance? Thx,

-Chris

@lattner
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lattner commented Dec 19, 2006

Devang, any news on this one?

@lattner
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lattner commented Jan 7, 2007

Reduced testcase:

static const char foo[] = "\0abc";

int test(int J) {
int i;
for (i = 0; i == 0 || foo[i] != 0; ++i)
;
return i;
}

void main() {
printf("%d\n", test(0));
}

$ llvm-gcc t.c -S -o - -O0 -emit-llvm | llvm-as | opt -mem2reg -instcombine -indvars -adce | lli
0
$ gcc t.c
$ ./a.out
4

@lattner
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lattner commented Jan 7, 2007

Fixed. Testcase here: Transforms/IndVarsSimplify/2007-01-06-TripCount.ll

Patch here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070101/042170.html

-Chris

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021
clementval pushed a commit to clementval/llvm-project that referenced this issue Jan 17, 2022
@jeanPerier
Add location to some lowering TODOs that missed it (llvm#1387)
94aa416 
Improve TODO messages:

- `genLocation()` was used instead of `getCurrentLocation` in many places that
intended to print the current location. Rename it to `genUnknownLocation()` to avoid
confusion in the futur.
- Thread/Compute the location when possible instead of using TODO_NOLOC.
- Ensure the location is set when entering not yet implemented
constructs (toLocation() might return an empty location in case the
construct is the first of an anonymous program)
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