VM value out of range only with --cpu:arm

[ajusa]

Nim VM has value out of range when raising 2^31 and --cpu:arm

Example

import math
const a: uint32 = (2 ^ 31).uint32

Current Output

stack trace: (most recent call last)
/home/ajusa/Documents/tmp/hid.nim(2, 22)
/home/ajusa/.choosenim/toolchains/nim-#devel/lib/pure/math.nim(1112, 16) ^
/home/ajusa/.choosenim/toolchains/nim-#devel/lib/system/arithmetics.nim(468, 9) *=
/home/ajusa/.choosenim/toolchains/nim-#devel/lib/system/arithmetics.nim(468, 9) Error: unhandled exception: value out of range

Expected Output

(Blank, since this should compile correctly). It does compile correctly without --cpu:arm

Additional Information

I think that 2^30 works just fine, it was 2^31 that gave me an issue in the project I am working on.

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-08-04
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: 6563a685c1076cb5342f65d192b8f2a983220dba
active boot switches: -d:release

Also had the same issue on Nim 1.4.4, shouldn't be a regression.

[demotomohiro]

I don't think this is a bug.

With --cc:arm option, sizeof(int) become 4 bytes and it seems sizeof(int) in VM also become 4 bytes. Type of literals 2 and 31 in your code is int type. Expression 2 ^ 31 calls ^ operator defined in math module with the first argument as int type and calculate the result as int type. 2 ^ 31 cannot be represent with 32bit signed int value. (2 ^ 63 cannot be repesent with 64bit signed int value and echo (2 ^ 63).uint64 cause overflow error on 64bit CPU)

Following code result in overflow on my raspberry pi3:

import std/math

static:
  echo hostCPU

echo "sizeof int:", sizeof(int)
echo 2^31

output:

[alarm@rasp proj]$ nim c intsizetest.nim
Hint: used config file '/home/alarm/src/nim-1.5.1/config/nim.cfg' [Conf]
Hint: used config file '/home/alarm/src/nim-1.5.1/config/config.nims' [Conf]
.............................................................
arm
CC: intsizetest.nim
Hint:  [Link]
Hint: gc: refc; opt: none (DEBUG BUILD, `-d:release` generates faster code)
30551 lines; 2.312s; 25.977MiB peakmem; proj: /home/alarm/proj/intsizetest.nim; out: /home/alarm/proj/intsizetest [SuccessX]
[alarm@rasp proj]$ ./intsizetest
sizeof int:4
/home/alarm/proj/intsizetest.nim(7) intsizetest
/home/alarm/src/nim-1.5.1/lib/pure/math.nim(1112) ^
/home/alarm/src/nim-1.5.1/lib/system/arithmetics.nim(468) *=
/home/alarm/src/nim-1.5.1/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: over- or underflow [OverflowDefect]

When I write the expression as first argument is uint32 type like 2.uint32 ^ 31, it works without overflow on my raspberry pi 3.

[ajusa]

2.uint32 ^ 31

I see, that would explain the issue along with the correct way of doing things. I was unaware that setting the CPU to arm makes integers 32bit, including in the VM. Closing :smile: