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C++: remove some side-effect instructions

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This leverages the existing alias analysis to identify functions which
have no reads or writes of the AllAliasedMemory virtual variable, and
therefore have no global side effects. A recursion over the call graph
identifies functions which have no indirect global side effects, and
calls to those functions have their global side effect instructions
removed.
This commit is contained in:
Robert Marsh
2022-07-29 10:38:13 -04:00
parent 4ae218729a
commit d324969d68
6 changed files with 325 additions and 508 deletions
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18
cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/internal/SSAConstruction.qll
View File

@@ -11,8 +11,11 @@ private import NewIR
private class OldBlock = Reachability::ReachableBlock;
private class OldInstruction = Reachability::ReachableInstruction;
private class OldInstruction extends Reachability::ReachableInstruction {
OldInstruction() { not Elim::removeableSideEffect(this)}
}
private import SideEffectElimination as Elim
import Cached
cached
@@ -58,7 +61,9 @@ private module Cached {
cached
predicate hasInstruction(TStageInstruction instr) {
instr instanceof TRawInstruction and instr instanceof OldInstruction
instr instanceof TRawInstruction and
instr instanceof OldInstruction and
not Elim::removeableSideEffect(instr)
or
instr = phiInstruction(_, _)
or
@@ -382,7 +387,14 @@ private module Cached {
(
if Reachability::isInfeasibleInstructionSuccessor(oldInstruction, kind)
then result = unreachedInstruction(instruction.getEnclosingIRFunction())
else result = getNewInstruction(oldInstruction.getSuccessor(kind))
else
if Elim::removeableSideEffect(oldInstruction.getSuccessor(kind))
then
// the only removed nodes are side-effect writes, but those may have Chi nodes
// skip to the following instruction in the old IR, which won't be removed
// if we start skipping specific side effects, this may no longer hold
result = getNewInstruction(oldInstruction.getSuccessor(kind).getSuccessor(kind))
else result = getNewInstruction(oldInstruction.getSuccessor(kind))
)
)
or

1
cpp/ql/lib/semmle/code/cpp/ir/implementation/internal/TInstruction.qll
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@@ -34,6 +34,7 @@ newtype TInstruction =
AliasedSSA::SSA::hasPhiInstruction(blockStartInstr, memoryLocation)
} or
TAliasedSsaChiInstruction(TRawInstruction primaryInstruction) {
not Elim::removeableSideEffect(primaryInstruction) and
AliasedSSA::SSA::hasChiInstruction(primaryInstruction)
} or
TAliasedSsaUnreachedInstruction(IRFunctionBase irFunc) {

1
cpp/ql/lib/semmle/code/cpp/ir/implementation/internal/TInstructionInternal.qll
View File

@@ -2,3 +2,4 @@ import semmle.code.cpp.ir.internal.IRCppLanguage as Language
import semmle.code.cpp.ir.implementation.raw.internal.IRConstruction as IRConstruction
import semmle.code.cpp.ir.implementation.unaliased_ssa.internal.SSAConstruction as UnaliasedSsa
import semmle.code.cpp.ir.implementation.aliased_ssa.internal.SSAConstruction as AliasedSSA
import semmle.code.cpp.ir.implementation.aliased_ssa.internal.SideEffectElimination as Elim