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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Loop reroller assertion failure for daxpy loop"
href="http://llvm.org/bugs/show_bug.cgi?id=18290">18290</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Loop reroller assertion failure for daxpy loop
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>opt
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dpeixott@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11756" name="attach_11756" title="test.ll">attachment 11756</a> <a href="attachment.cgi?id=11756&action=edit" title="test.ll">[details]</a></span>
test.ll
Description
-------------------------------------------------------------------
The loop reroller causes an assertion failure for this loop when
compiling for 32-bit targets:
#define REAL float
void daxpy_ur(int n,REAL da,REAL *dx,REAL *dy,int m)
{
for (int i = m; i < n; i = i + 4)
{
dy[i] = dy[i] + da*dx[i];
dy[i+1] = dy[i+1] + da*dx[i+1];
dy[i+2] = dy[i+2] + da*dx[i+2];
dy[i+3] = dy[i+3] + da*dx[i+3];
}
}
The assertion failure is:
opt: include/llvm/Support/Casting.h:239: typename llvm::cast_retty<X,
Y*>::ret_type llvm::cast(Y*) [with X = llvm::PHINode, Y = llvm::Value]:
Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
0 opt 0x00000000013f4802 llvm::sys::PrintStackTrace(_IO_FILE*) +
34
1 opt 0x00000000013f5cea
2 libpthread.so.0 0x00007f315825b8f0
3 libc.so.6 0x00007f31570ffa75 gsignal + 53
4 libc.so.6 0x00007f31571035c0 abort + 384
5 libc.so.6 0x00007f31570f8941 __assert_fail + 241
6 opt 0x0000000000f0fabd
7 opt 0x0000000001145657
llvm::LPPassManager::runOnFunction(llvm::Function&) + 1287
8 opt 0x0000000001384b28
llvm::FPPassManager::runOnFunction(llvm::Function&) + 568
9 opt 0x0000000001384c0b
llvm::FPPassManager::runOnModule(llvm::Module&) + 43
10 opt 0x000000000138463c
llvm::legacy::PassManagerImpl::run(llvm::Module&) + 892
11 opt 0x000000000058ee9e main + 6110
12 libc.so.6 0x00007f31570eac4d __libc_start_main + 253
13 opt 0x0000000000580909
Stack dump:
0. Program arguments: opt -loop-reroll -S -debug-only=loop-reroll
1. Running pass 'Function Pass Manager' on module '<stdin>'.
2. Running pass 'Loop Pass Manager' on function '@daxpy_ur'
3. Running pass 'Reroll loops' on basic block '%for.body'
Steps to reproduce
-------------------------------------------------------------------
1. Put the C program above in small.c
2. Compile
$ clang -target arm-none-linux -mfloat-abi=soft small.c -S -o- -O1 -emit-llvm
-o test.ll
$ opt -loop-reroll -S -debug-only=loop-reroll < test.ll
I also can reproduce the assertion using -target i386-none-linux.
I've attached the bitcode to the bug.
Analysis
-------------------------------------------------------------------
The assertion comes from LoopReroll::reroll() function when
expanding the the code for the new induction variable. The new
induction varible is cast to a phi node, but the value produced by
the scev expander is not actually a phi node.
PHINode *NewIV =
cast<PHINode>(Expander.expandCodeFor(H, IV->getType(),
Header->begin()));
The scev expander produces an induction variable that looks like
this:
%indvar = phi i32 [ %indvar.next, %for.body ], [ 0, %entry ]
%i.055 = phi i32 [ %add27, %for.body ], [ %m, %entry ]
%0 = add i32 %m, %indvar
The '%0' value is what is returned from the scev expander. It uses a
phi node, as an operand but is actually an add instruction.
It looks like the cast to a phi-node is only needed to get the
backedge value to use for the end-of-loop test. I made a quick
change to use a comparison of "NewIV == (IterCount-1)" instead of
"NextIV == IterCount". It works for this example, but it causes some
make-check failures for loop-rerolling and I'm not sure it is the
correct fix.</pre>
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