[llvm-bugs] [Bug 35233] New: fetestexcept reordered with floating point operation

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https://bugs.llvm.org/show_bug.cgi?id=35233

            Bug ID: 35233
           Summary: fetestexcept reordered with floating point operation
           Product: clang
           Version: 4.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: peter at trailofbits.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19381
  --> https://bugs.llvm.org/attachment.cgi?id=19381&action=edit
C++11 file exhibiting the bug

A static_cast of a uint64_t to a double on Clang 4.0 and AArch64 is reordered
with a fettestexcept, resulting in the wrong floating point state being tested.

Example output from GCC 6.0: https://godbolt.org/g/pbAW4J

Here's Clang's output:

oot at mcsema-aarch64:~/remill# ../remill-build/libraries/llvm/bin/clang++
-std=c++11 -O3 -c /tmp/convert.cc -S -o /tmp/convert.S
root at mcsema-aarch64:~/remill# cat /tmp/convert.S
    .text
    .file    "/tmp/convert.cc"
    .globl    _Z9to_doubleR5flagsm
    .p2align    2
    .type    _Z9to_doubleR5flagsm, at function
_Z9to_doubleR5flagsm:                   // @_Z9to_doubleR5flagsm
// BB#0:
    stp    x20, x19, [sp, #-32]!   // 8-byte Folded Spill
    mov     x19, x0
    orr    w0, wzr, #0x1f
    stp    x29, x30, [sp, #16]     // 8-byte Folded Spill
    add    x29, sp, #16            // =16
    mov     x20, x1
    bl    feclearexcept
    orr    w0, wzr, #0x1f
    bl    fetestexcept
    tbz    w0, #4, .LBB0_2
// BB#1:
    orr    w8, wzr, #0x1
    str        w8, [x19]
.LBB0_2:
    tbz    w0, #2, .LBB0_4
// BB#3:
    orr    w8, wzr, #0x1
    str    w8, [x19, #4]
.LBB0_4:
    tbz    w0, #3, .LBB0_6
// BB#5:
    orr    w8, wzr, #0x1
    str    w8, [x19, #8]
.LBB0_6:
    ucvtf    d0, x20
    tbz    w0, #0, .LBB0_8
// BB#7:
    orr    w8, wzr, #0x1
    str    w8, [x19, #12]
.LBB0_8:
    ldp    x29, x30, [sp, #16]     // 8-byte Folded Reload
    ldp    x20, x19, [sp], #32     // 8-byte Folded Reload
    ret
.Lfunc_end0:
    .size    _Z9to_doubleR5flagsm, .Lfunc_end0-_Z9to_doubleR5flagsm

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