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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - sign mismatch for getArithmeticInstrCost() definitions/usage"
href="https://bugs.llvm.org/show_bug.cgi?id=43591">43591</a>
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<th>Summary</th>
<td>sign mismatch for getArithmeticInstrCost() definitions/usage
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Transformation Utilities
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>Follow-up to <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - Assert "Non-zero loop cost expected" failed (since "[SLP] avoid reduction transform on patterns that the backend can load-combine")"
href="show_bug.cgi?id=43582">bug 43582</a>:
In class TargetTransformInfo::Model final : public TargetTransformInfo::Concept
(in TargetTransformInfo.h), we have:
/// This is an approximation of reciprocal throughput of a math/logic op.
/// A higher cost indicates less expected throughput.
/// From Agner Fog's guides, reciprocal throughput is "the average number of
/// clock cycles per instruction when the instructions are not part of a
/// limiting dependency chain."
/// Therefore, costs should be scaled to account for multiple execution units
/// on the target that can process this type of instruction. For example, if
/// there are 5 scalar integer units and 2 vector integer units that can
/// calculate an 'add' in a single cycle, this model should indicate that the
/// cost of the vector add instruction is 2.5 times the cost of the scalar
/// add instruction.
/// \p Args is an optional argument which holds the instruction operands
/// values so the TTI can analyze those values searching for special
/// cases or optimizations based on those values.
int getArithmeticInstrCost(
unsigned Opcode, Type *Ty, OperandValueKind Opd1Info = OK_AnyValue,
OperandValueKind Opd2Info = OK_AnyValue,
OperandValueProperties Opd1PropInfo = OP_None,
OperandValueProperties Opd2PropInfo = OP_None,
ArrayRef<const Value *> Args = ArrayRef<const Value *>()) const;
In class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> (in
BasicTTIImpl.h), we have:
unsigned getArithmeticInstrCost(
unsigned Opcode, Type *Ty,
TTI::OperandValueKind Opd1Info = TTI::OK_AnyValue,
TTI::OperandValueKind Opd2Info = TTI::OK_AnyValue,
TTI::OperandValueProperties Opd1PropInfo = TTI::OP_None,
TTI::OperandValueProperties Opd2PropInfo = TTI::OP_None,
ArrayRef<const Value *> Args = ArrayRef<const Value *>()) {
----------------------------------------------------------------------------
We're returning a signed value in the first function even though a reciprocal
throughput shouldn't ever be negative. The LoopVectorizer assumes the value is
unsigned. The SLPVectorizer uses these values to compare relative perf and uses
subtraction of signed values when comparing scalar and vector sequences.
Both of these interpretations can't be right?</pre>
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