[LLVMbugs] [Bug 1097] NEW: Strength reduction misses IV reuse opportunities on 64-bit targets

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1097

           Summary: Strength reduction misses IV reuse opportunities on 64-
                    bit targets
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Loop Optimizer
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: djg at cray.com


The strength reduction pass is causing the code generator to miss opportunities
for using complex addressing modes.

For this test case:

define void %foo(double* %y) {
entry:
        br label %bb

bb:
        %i = phi i64 [ 0, %entry ], [ %k, %bb ]
        %j = getelementptr double* %y, i64 %i
        store double 0.000000e+00, double* %j
        %k = add i64 %i, 1
        %n = icmp eq i64 %k, 0
        br bool %n, label %return, label %bb

return:
        ret void
}

The strength reduction pass changes the output on x86-64 to have two induction
variables:

        %iv..inc = add i64 %iv., 1
        ...
        %iv.1.inc = add i64 %iv.1, 8

which causes the codegen to produce this:

.LBB1_1:        #bb
        movq $0, (%rdi)
        addq $8, %rdi
        incq %rax
        cmpq $0, %rax
        jne .LBB1_1     #bb

instead of this:

.LBB1_1:        #bb
        movq $0, (%rdi,%rax,8)
        incq %rax
        cmpq $0, %rax
        jne .LBB1_1     #bb


Here's a patch that fixes this.

Index: LoopStrengthReduce.cpp
===================================================================
RCS file: /var/cvs/llvm/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp,v
retrieving revision 1.104
diff -u -r1.104 LoopStrengthReduce.cpp
--- LoopStrengthReduce.cpp
+++ LoopStrengthReduce.cpp
@@ -893,7 +893,7 @@
       if (unsigned(abs(SInt)) < Scale || (SInt % Scale) != 0)
         continue;
       std::map<SCEVHandle, IVsOfOneStride>::iterator SI =
-        IVsByStride.find(SCEVUnknown::getIntegerSCEV(SInt/Scale, Type::Int32Ty));
+        IVsByStride.find(SCEVUnknown::getIntegerSCEV(SInt/Scale, UIntPtrTy));
       if (SI == IVsByStride.end())
         continue;
       for (std::vector<IVExpr>::iterator II = SI->second.IVs.begin(),



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