[issue15495] enable type truncation warnings for gcc builds

[Martin Panter]

Martin Panter added the comment:

Issue 23545 has a patch with some comments from me about adding other warnings 
via “autoconf foo”, so you could copy from that if you want.

The few warnings that I glanced at do not look troublesome. But maybe it is 
worth working around them to see other warnings; I dunno. Adding explicit casts 
can sometimes hide other bugs. Anyway, here is a random selection of some 
warnings:

Modules/sha256module.c:198:44: warning: conversion to ‘SHA_INT32 {aka unsigned 
int}’ from ‘long unsigned int’ may alter its value [-Wconversion]
 RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],48,0x19a4c116);
^
Modules/sha256module.c:145:11: note: in definition of macro ‘RND’
  t0 = h + Sigma1(e) + Ch(e, f, g) + ki + W[i];   \
   ^
Modules/resource.c:19:60: warning: conversion to ‘double’ from ‘__suseconds_t 
{aka long int}’ may alter its value [-Wconversion]
 #define doubletime(TV) ((double)(TV).tv_sec + (TV).tv_usec * 0.01)
^
./Include/tupleobject.h:62:75: note: in definition of macro ‘PyTuple_SET_ITEM’
 #define PyTuple_SET_ITEM(op, i, v) (((PyTupleObject *)(op))->ob_item[i] = v)
   ^
Modules/resource.c:82:5: note: in expansion of macro ‘PyStructSequence_SET_ITEM’
 PyStructSequence_SET_ITEM(result, 0,
 ^
Modules/resource.c:83:40: note: in expansion of macro ‘doubletime’
 PyFloat_FromDouble(doubletime(ru.ru_utime)));
^~
Modules/cjkcodecs/cjkcodecs.h:155:27: warning: conversion to ‘unsigned char’ 
from ‘int’ may alter its value [-Wconversion]
 do { ((*outbuf)[1]) = (c); } while (0)
   ^
Modules/cjkcodecs/_codecs_kr.c:58:13: note: in expansion of macro ‘OUTBYTE2’
 OUTBYTE2((code & 0xFF) | 0x80);
 ^~~~
Modules/_ctypes/cfield.c:439:15: warning: conversion to ‘short int’ from ‘int’ 
may alter its value [-Wconversion]
 v >>= (sizeof(v)*8 - NUM_BITS(size));   \
   ^
Modules/_ctypes/cfield.c:594:5: note: in expansion of macro ‘GET_BITFIELD’
 GET_BITFIELD(val, size);
 ^~~~

--
nosy: +martin.panter

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[issue15495] enable type truncation warnings for gcc builds

[STINNER Victor]

Changes by STINNER Victor :


--
nosy: +haypo

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[issue15495] enable type truncation warnings for gcc builds

[Jeremy Kloth]

Jeremy Kloth added the comment:

> I'm getting more than thousand warnings:
>
> $ CFLAGS="-Wconversion -Wno-sign-conversion" ./configure
> $ LC_ALL=C make -s -j8 2>&1 | tee log
> $ grep Wconversion log | wc -l
> 1163

My Win64 buildbot currentlyhas 544 conversion warnings, but it seems
something has recently changed with OpenSSL thus causing over 2100
total warnings.

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nosy: +jeremy.kloth

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[issue15495] enable type truncation warnings for gcc builds

[Christian Heimes]

Christian Heimes added the comment:

I'm getting more than thousand warnings:

$ CFLAGS="-Wconversion -Wno-sign-conversion" ./configure
$ LC_ALL=C make -s -j8 2>&1 | tee log
$ grep Wconversion log | wc -l
1163

platform: Ubuntu 12.04 x86_64
gcc: 4.6.3

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[issue15495] enable type truncation warnings for gcc builds

[Mark Dickinson]

Mark Dickinson added the comment:

How many extra warnings do you get by adding these flags (e.g., just by doing 
'export CFLAGS= ...' before building)?  It might be useful to see a sampling of 
those warnings.

The addition of these flags should be conditional on gcc's version being >= 
4.3:  gcc 4.2 apparently has a different meaning for -Wconversion (to do with 
implicit conversions when passing function arguments), and generates crazy 
numbers of warnings on my OS X 10.6 machine (which comes with gcc 4.2).

Why '-Wno-sign-conversion'?  Is fixing all the places that have implicit sign 
conversions a reasonable goal, or are there just too many of those?

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[issue15495] enable type truncation warnings for gcc builds

[Antoine Pitrou]

Changes by Antoine Pitrou :


--
nosy: +mark.dickinson

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[issue15495] enable type truncation warnings for gcc builds

[Jeremy Kloth]

New submission from Jeremy Kloth:

It would seem that enabling GCC's type conversion/truncation warnings would be 
a good thing for Python builds.

This would bring GCC builds in line with MSVC builds and reduce the burden on 
the Windows developers in fixing them.  Also, it would bring attention to those 
issues by the developer who writes code and would hopefully understand the 
truncation issues at hand.

The CFLAGS required for this are '-Wconversion -Wno-sign-conversion'.

I unfortunately do not know the autoconf foo needed to implement this 
suggestion, however.  Perhaps it should only be enabled for debug builds as 
well to eliminate excess noise for other builds.  Or at least enabled on some 
of the buildbots.

--
components: Build
messages: 166851
nosy: jkloth
priority: normal
severity: normal
status: open
title: enable type truncation warnings for gcc builds
type: enhancement
versions: Python 3.3, Python 3.4

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