Bug 92220

Summary: -Wconversion generates a false warning for modulo expression when the modulus has smaller type
Product: gcc Reporter: John Simon <gcc>
Component: cAssignee: Not yet assigned to anyone <unassigned>
Status: NEW ---    
Severity: normal CC: xry111
Priority: P3 Keywords: diagnostic
Version: 9.2.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed: 2020-01-29 00:00:00

Description John Simon 2019-10-25 04:50:37 UTC 
```
[]$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --with-system-zlib --with-isl --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-plugin --enable-install-libiberty --with-linker-hash-style=gnu --enable-gnu-indirect-function --enable-multilib --disable-werror --enable-checking=release --enable-default-pie --enable-default-ssp --enable-cet=auto
Thread model: posix
gcc version 9.2.0 (GCC) 

[]$ cat a.c
int f(long long a,int b){
	return a%b;
}

[]$ gcc -Wall -Wconversion a.c -c
a.c: In function ‘f’:
a.c:2:10: warning: conversion from ‘long long int’ to ‘int’ may change value [-Wconversion]
    2 |  return a%b;
      |         ~^~
```

The specification for `-Wconversion` is "Warn for implicit conversions that may alter a value.". In this case the result is guaranteed to be convertible to the type of `b` (`int`) without changing the value.
Comment 1 Andrew Pinski 2019-10-25 05:40:56 UTC 
No i think there is one case where what you said is incorrect. When both are INT_MIN.
Comment 2 Richard Biener 2019-10-25 08:28:27 UTC 
*** Bug 92219 has been marked as a duplicate of this bug. ***
Comment 3 John Simon 2019-10-25 08:49:41 UTC 
(In reply to Andrew Pinski from comment #1)
> No i think there is one case where what you said is incorrect. When both are
> INT_MIN.

In this case the result will be 0.
Comment 4 Martin Liška 2020-01-29 14:41:09 UTC 
Confirmed.
Comment 5 Xi Ruoyao 2023-06-24 13:27:40 UTC 
Unfortunately the value range info is not available in the frontend :(.
Comment 6 Xi Ruoyao 2023-06-24 13:30:59 UTC 
(In reply to Xi Ruoyao from comment #5)
> Unfortunately the value range info is not available in the frontend :(.

Here "value range info" means the info from the VRP pass.  For this case we can specially check MOD_EXPR, but doing so won't help with the general cases like "return a % b / 2;".