[llvm-bugs] [Bug 34336] New: Loop-idiom recognition rewrites a loop into memset instead of unrolling it when using LTO

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34336

            Bug ID: 34336
           Summary: Loop-idiom recognition rewrites a loop into memset
                    instead of unrolling it when using LTO
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: phosek at chromium.org
                CC: llvm-bugs at lists.llvm.org

This was observed when building our libc with LTO for aarch64 (we haven't seen
the same issue on x86_64), specifically in _dl_start function.

The source snippet is the following:

#define DYN_CNT 32
size_t i, dyn[DYN_CNT];
for (i = 0; i < DYN_CNT; i++)
    dyn[i] = 0;

In the non-LTO build, this loop is unrolled as expected:

   caa24:       ad0003e0        stp     q0, q0, [sp]
   caa28:       ad0103e0        stp     q0, q0, [sp,#32]
   caa2c:       ad0203e0        stp     q0, q0, [sp,#64]
   caa30:       ad0303e0        stp     q0, q0, [sp,#96]
   caa34:       ad0403e0        stp     q0, q0, [sp,#128]
   caa38:       ad0503e0        stp     q0, q0, [sp,#160]
   caa3c:       ad0603e0        stp     q0, q0, [sp,#192]
   caa40:       ad0703e0        stp     q0, q0, [sp,#224]

In the LTO build, the loops is turned into a PLT call:

   e1814:       940015a3        bl      e6ea0 <memset at plt>

Since this code is at the beginning of _dl_start before any relocations have
been performed, this causes segfault.

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