[LLVMbugs] [Bug 8883] New: SCEV doing a poor job simplifying constant expressions

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8883

           Summary: SCEV doing a poor job simplifying constant expressions
           Product: libraries
           Version: 1.0
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: clattner at apple.com
                CC: llvmbugs at cs.uiuc.edu


This code:


void fill(Iterator first, Iterator last, T value) {
        while (first != last) *first++ = value;
}
char X[1000];
void foo() {
  fill(X, X+1000, 0);
}

Compiles with: clang t.cpp -S -o - -emit-llvm -O3

into:

...

while.body.i:                                     ; preds = %while.body.i,
%entry
  %indvar.i = phi i64 [ 0, %entry ], [ %indvar.next.i, %while.body.i ]
  %first.addr.05.i = getelementptr [1000 x i8]* @X, i64 0, i64 %indvar.i
  store i8 0, i8* %first.addr.05.i, align 1, !tbaa !0
  %indvar.next.i = add i64 %indvar.i, 1
  %exitcond = icmp eq i64 %indvar.next.i, ptrtoint (i8* getelementptr ([1000 x
i8]* @X, i64 1, i64 sub (i64 0, i64 ptrtoint ([1000 x i8]* @X to i64))) to i64)
  br i1 %exitcond, label %_Z4fillIPciEvT_S1_T0_.exit, label %while.body.i

Notice the poor icmp, which should be comparing the indvar against 1000. SCEV
should simplify this away.  This can also be seen in
simple_types_constant_folding.llvm.bc in
Nehalem:SingleSource/Benchmarks/Adobe-C++ where SCEV is expanding an expression
for the loop-idiom pass, resulting in this crazy memset:

call void @llvm.memset.p0i8.i64(i8* getelementptr inbounds ([8000 x i8]*
@data8, i64 0, i64 0), i8 %11, i64 ptrtoint (i8* getelementptr ([8000 x i8]*
@data8, i64 1, i64 sub (i64 0, i64 ptrtoint ([8000 x i8]* @data8 to i64))) to
i64), i32 16, i1 false)


Codegen is able to simplify the constant to "8000", but the expression
simplification logic in SCEV should do this.  This may be just a matter of
using lib/Analysis/ConstantFolding APIs with target data in scev.

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