[llvm-bugs] [Bug 30188] New: SimplifyCFG blocks SROA optimization

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30188

            Bug ID: 30188
           Summary: SimplifyCFG blocks SROA optimization
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: danielcdh at gmail.com
                CC: davidxl at google.com, james.molloy at arm.com,
                    llvm-bugs at lists.llvm.org, wmi at google.com
    Classification: Unclassified

struct b {
  int x_;
};

struct a {
  int l_;
  struct b *data_;
};

int BinarySearch(struct a *input, struct b t) {
  if (input->l_ > 0) {
    int low = 0;
    int high = input->l_;
    while (high != low + 1) {
      int mid = (high + low) / 2;
      if (input->data_[mid].x_ > t.x_)
        high = mid;
      else
        low = mid;
    }
    return low;
  }
  return -1;
}

The bug issue was introduced by
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160822/384231.html

Without the patch, the inner loop x86 code (compiled at -O2):

        leal    (%rcx,%rax), %edx
        movl    %edx, %edi
        shrl    $31, %edi
        addl    %edx, %edi
        sarl    %edi
        movslq  %edi, %rdx
        cmpl    %esi, (%r8,%rdx,4)
        cmovlel %edx, %eax
        cmovgl  %edx, %ecx
        leal    1(%rax), %edx
        cmpl    %edx, %ecx
        jne     .LBB0_3

With the patch:

        addl    %ecx, %eax
        movl    %eax, %ecx
        shrl    $31, %ecx
        addl    %eax, %ecx
        sarl    %ecx
        movslq  %ecx, %rax
        cmpl    %esi, (%rdx,%rax,4)
        movq    %r9, %rcx
        cmovgq  %r8, %rcx
        movl    %eax, (%rcx)
        movl    -8(%rsp), %ecx
        movl    -4(%rsp), %eax
        leal    1(%rax), %edi
        cmpl    %edi, %ecx
        jne     .LBB0_3

2 more memory store instructions are generated.

This is because when simplifycfg combines 2 store address into a select, SROA
can no longer find the pairing for instruction addresses between load and
store, thus the store cannot be removed.

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