[llvm-bugs] [Bug 35681] New: [x86] large constant offsets used for memory accesses in loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35681

            Bug ID: 35681
           Summary: [x86] large constant offsets used for memory accesses
                    in loop
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Forking this off from bug 28343:

$ cat badmax.c
void maxArray(double* __restrict x, double* __restrict y) {
    for (int i = 0; i < 65536; i++) x[i] = ((y[i] > x[i]) ? y[i] : x[i]);
}

Or as IR compiled as:
$ ./clang badmax.c -S -o - -O2 -fno-unroll-loops -emit-llvm

define void @maxArray(double* noalias nocapture %x, double* noalias nocapture
readonly %y) {
entry:
  br label %vector.body

vector.body:
  %index = phi i64 [ 0, %entry ], [ %index.next, %vector.body ]
  %0 = getelementptr inbounds double, double* %y, i64 %index
  %1 = bitcast double* %0 to <2 x double>*
  %wide.load = load <2 x double>, <2 x double>* %1, align 8
  %2 = getelementptr inbounds double, double* %x, i64 %index
  %3 = bitcast double* %2 to <2 x double>*
  %wide.load21 = load <2 x double>, <2 x double>* %3, align 8
  %4 = fcmp ogt <2 x double> %wide.load, %wide.load21
  %5 = select <2 x i1> %4, <2 x double> %wide.load, <2 x double> %wide.load21
  %6 = bitcast double* %2 to <2 x double>*
  store <2 x double> %5, <2 x double>* %6, align 8
  %index.next = add i64 %index, 2
  %7 = icmp eq i64 %index.next, 65536
  br i1 %7, label %for.cond.cleanup, label %vector.body

for.cond.cleanup:
  ret void
}

-----------------------------------------------------------------------------

$ ./llc -o - badmax.ll -mtriple=x86_64
        movq    $-524288, %rax          # imm = 0xFFF80000
        .p2align        4, 0x90
.LBB0_1:
        movupd  524288(%rsi,%rax), %xmm0
        movupd  524288(%rdi,%rax), %xmm1
        maxpd   %xmm1, %xmm0
        movupd  %xmm0, 524288(%rdi,%rax)
        addq    $16, %rax
        jne     .LBB0_1                      
        retq

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