http://llvm.org/bugs/show_bug.cgi?id=14431
Bug #: 14431
Summary: Missed opportunity for dead store elimination
Product: libraries
Version: trunk
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: Global Analyses
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
While looking at some generated code from a bootstrap build.
I noticed this sequence of code
SDValue Ops[6];
Ops[0] = getRoot();
Ops[1] = getValue(I.getArgOperand(0));
Ops[2] = getValue(I.getArgOperand(1));
Ops[3] = getValue(I.getArgOperand(2));
Ops[4] = DAG.getSrcValue(I.getArgOperand(0));
Ops[5] = DAG.getSrcValue(F);
Res = DAG.getNode(ISD::INIT_TRAMPOLINE, dl, MVT::Other, Ops, 6);
Generated this assembly:
movq $0, -2272(%rbp)
movl $0, -2264(%rbp)
movq $0, -2256(%rbp)
movl $0, -2248(%rbp)
movq $0, -2240(%rbp)
movl $0, -2232(%rbp)
movq $0, -2224(%rbp)
movl $0, -2216(%rbp)
movq $0, -2208(%rbp)
movl $0, -2200(%rbp)
movq $0, -2192(%rbp)
movl $0, -2184(%rbp)
movq %r12, %rdi
callq __ZN4llvm19SelectionDAGBuilder7getRootEv
movq %rax, -2272(%rbp)
movl %edx, -2264(%rbp)
movl 48(%rbx), %eax
negq %rax
leaq (%rax,%rax,2), %rax
movq (%rbx,%rax,8), %rsi
movq %r12, %rdi
callq __ZN4llvm19SelectionDAGBuilder8getValueEPKNS_5ValueE
movq %rax, -2256(%rbp)
movl %edx, -2248(%rbp)
movl 48(%rbx), %eax
subq %rax, %r15
leaq (%r15,%r15,2), %rax
movq (%rbx,%rax,8), %rsi
movq %r12, %rdi
callq __ZN4llvm19SelectionDAGBuilder8getValueEPKNS_5ValueE
movl $2, %ecx
movq %rax, -2240(%rbp)
movl %edx, -2232(%rbp)
movl 48(%rbx), %eax
subq %rax, %rcx
leaq (%rcx,%rcx,2), %rax
movq (%rbx,%rax,8), %rsi
movq %r12, %rdi
callq __ZN4llvm19SelectionDAGBuilder8getValueEPKNS_5ValueE
movq %rax, -2224(%rbp)
movl %edx, -2216(%rbp)
movl 48(%rbx), %eax
negq %rax
leaq (%rax,%rax,2), %rax
movq (%rbx,%rax,8), %rsi
movq 408(%r12), %rdi
callq __ZN4llvm12SelectionDAG11getSrcValueEPKNS_5ValueE
movq %rax, -2208(%rbp)
movl %edx, -2200(%rbp)
movq 408(%r12), %rdi
movq %r14, %rsi
callq __ZN4llvm12SelectionDAG11getSrcValueEPKNS_5ValueE
movq $0, -4832(%rbp) ## 8-byte Folded Spill
leaq -2272(%rbp), %r9
movq %rax, -2192(%rbp)
movl %edx, -2184(%rbp)
You'll see at the start of the assembly there are 12 stores of 0 to initialize
the SDValue array. But each of those values are overwritten by the getRoot,
getValue, getSrcValue calls. Dead store elimination should have been able to
remove the stores of 0.
As far as I can tell this didn't happen because alias analysis thinks that
getRoot, getValue, and getSrcValue could reference this SDValue array.
This seems to be a problem with this code in
BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, const Location &Loc)
// If the pointer is to a locally allocated object that does not escape,
// then the call can not mod/ref the pointer unless the call takes the
pointer
// as an argument, and itself doesn't capture it.
if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
isNonEscapingLocalObject(Object)) {
In this case the alloca for the SDValue array does escape through the call to
DAG.getNode so the isNonEscapingLocalObject returns false. But the CallSites in
question here are for getRoot, getValue, getSrcValue which it doesn't into.
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