GCC/Clang O2 and O3 - Some shifts fail in release mode

[mratsim]

import ../src/mpint

let
  x: uint = 6905570704192331'u
  y: uint = 6821857284170478289'u
  z = x shl y

echo "x: " & $x
echo "y: " & $y
echo "z: " & $z

echo "\n"

let
  tx = cast[MpUint[64]](x)
  tz = cast[uint](x shl y)

echo "tz: " & $tz

I get the correct 1236503728129179648 in normal and 0 in release.

[mratsim]

Trail log of success-failures

x: 66515580246432666
y: 7675878813075937703
x: 56822405546580166
y: 898154221633338996
x: 6905570704192331
y: 6821857284170478289
    property_based.nim(155, 25): Check failed: cast[uint](tz) == x shl y
    x shl y was 1236503728129179648
x: 13650447464096448
y: 715920912865732181
    property_based.nim(155, 25): Check failed: cast[uint](tz) == x shl y
    x shl y was 16163141901279559680
x: 47816330350663417
y: 699151824707875437
x: 30481878820307362
y: 7761954026009152944
x: 66871851162843803
y: 1130862033762041323
x: 42742556254146342
y: 2980158593863262094
    property_based.nim(155, 25): Check failed: cast[uint](tz) == x shl y
    x shl y was 17764510940680257536
x: 41291219780227599
y: 362015085743059411
    property_based.nim(155, 25): Check failed: cast[uint](tz) == x shl y
    x shl y was 10460237674663378944
x: 36684717707990032
y: 6437488508587700026

[mratsim]

It seems like it's a GCC/Clang optimization issue, taking Nim release code:

• it doesn't happen when optimization is not done
• it happens with -O2 or -O3, instead of rolling over we get 0 sometimes with the primitive types (MpUint always roll over)

let
  x: uint = 6905570704192331'u
  y: uint = 6821857284170478289'u
  z = x shl y

echo "x: " & $x
echo "y: " & $y
echo "z: " & $z

echo "\n"

Given that we're using uint256, we may not have shifts bigger than 256 but it's something to check.

Other test case:

let
  x: uint = 43489028062581548'u
  y: uint = 18207
  z = x shl y

echo "x: " & $x
echo "y: " & $y
echo "z: " & $z

echo "\n"

[mratsim]

Shifts by negative value are also inconsistent between release and non-release

import ../src/mpint

let
  x: uint = 10'u
  y: int  = -2
  z = x shl y

echo "x: " & $x
echo "y: " & $y
echo "z: " & $z

echo "\n#############"

let
  tx = cast[MpUint[64]](x)
  tz = cast[uint](x shl y)

echo "mpint z: " & $tz

echo "\n############"

echo "shr: " & $(x shr -2)
let tzz = cast[uint](x shl -2)
echo "mpint shr: " & $tzz

Non-release:

x: 10
y: -2
z: 9223372036854775808

#############
mpint z: 9223372036854775808

############
shr: 0
mpint shr: 9223372036854775808

Release

x: 10
y: -2
z: 0

#############
mpint z: 9223372036854775808

############
shr: 73832
mpint shr: 0

[mratsim]

False alarm, behavior is undefined according to C standard ISO 9899:2011 6.5.7 Bit-wise shift operators:

The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.

[arnetheduck]

@mratsim maybe report this in the nim tracker, as nim prides itself in removing UB from the language..