[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Dong-hee Na  added the comment:

Thank you Pablo!!

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Change by Dong-hee Na :


--
versions: +Python 3.10 -Python 3.9

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[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Dong-hee Na  added the comment:


New changeset 86e322f1412be6c5a6c8fa8e73af68b99c42bb18 by Pablo Galindo in 
branch 'master':
bpo-40455: Fix gcc10+ warning about writing into a section of offset 0 
(GH-24384)
https://github.com/python/cpython/commit/86e322f1412be6c5a6c8fa8e73af68b99c42bb18


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[issue40455] GCC 10 compiler warnings

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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nosy: +pablogsal
nosy_count: 5.0 -> 6.0
pull_requests: +23200
pull_request: https://github.com/python/cpython/pull/24384

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[issue40455] GCC 10 compiler warnings

[Emmanuel Arias]

Emmanuel Arias  added the comment:

Hi, I've on my Debian (testing) the reported warning:

In function ‘assemble_lnotab’,
inlined from ‘assemble_emit’ at Python/compile.c:5706:25,
inlined from ‘assemble’ at Python/compile.c:6048:18:
Python/compile.c:5660:19: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]
 5660 | *lnotab++ = k;
  |  


gcc --version  
gcc (Debian 10.2.0-5) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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nosy: +eamanu

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[issue40455] GCC 10 compiler warnings

[hai shi]

Change by hai shi :


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nosy: +shihai1991

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[issue40455] GCC 10 compiler warnings

[STINNER Victor]

STINNER Victor  added the comment:

There are still warnings on compile.c:
https://buildbot.python.org/all/#/builders/448/builds/130

Python/compile.c:5660:19: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]
Python/compile.c:5660: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]
Python/compile.c:5660: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]

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[issue40455] GCC 10 compiler warnings

[Mark Dickinson]

Change by Mark Dickinson :


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nosy:  -mark.dickinson

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[issue40455] GCC 10 compiler warnings

[Andy Lester]

Andy Lester  added the comment:

For anyone following along, note that the PR above is different than the 
original suggestion.  The PR correctly sets x_size, not leaving it zero.

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[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Dong-hee Na  added the comment:


New changeset b88cd585d36d6285a5aeb0b6fdb70c134062181e by Dong-hee Na in branch 
'master':
bpo-40455: Remove gcc10 warning about x_digits (#19852)
https://github.com/python/cpython/commit/b88cd585d36d6285a5aeb0b6fdb70c134062181e


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[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Change by Dong-hee Na :


--
keywords: +patch
pull_requests: +19169
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/19852

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[issue40455] GCC 10 compiler warnings

[Dong-hee Na]

Dong-hee Na  added the comment:

@mark.dickinson @vstinner

I'd like to suggest this change.
There was no performance impact on my local machine.

--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -2852,7 +2852,8 @@ _PyLong_Frexp(PyLongObject *a, Py_ssize_t *e)
 {
 Py_ssize_t a_size, a_bits, shift_digits, shift_bits, x_size;
 /* See below for why x_digits is always large enough. */
-digit rem, x_digits[2 + (DBL_MANT_DIG + 1) / PyLong_SHIFT];
+digit rem;
+digit x_digits[2 + (DBL_MANT_DIG + 1) / PyLong_SHIFT] = {0,};
 double dx;
 /* Correction term for round-half-to-even rounding.  For a digit x,
"x + half_even_correction[x & 7]" gives x rounded to the nearest
@@ -2903,8 +2904,6 @@ _PyLong_Frexp(PyLongObject *a, Py_ssize_t *e)
 shift_digits = (DBL_MANT_DIG + 2 - a_bits) / PyLong_SHIFT;
 shift_bits = (DBL_MANT_DIG + 2 - a_bits) % PyLong_SHIFT;
 x_size = 0;
-while (x_size < shift_digits)
-x_digits[x_size++] = 0;
 rem = v_lshift(x_digits + x_size, a->ob_digit, a_size,
(int)shift_bits);
 x_size += a_size;

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nosy: +corona10

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[issue40455] GCC 10 compiler warnings

[Mark Dickinson]

Mark Dickinson  added the comment:

As to _why_ it's a false positive: at that point in the code, assuming 30-bit 
limbs and an IEEE 754 binary64 "double", we have (using Python notation for 
floor division)

a_size == 1 + (a_bits - 1) // 30

and

shift_digits == (a_bits - 55) // 30

from which it's clear that

shift_digits <= (a_bits - 1) // 30 < a_size

so a_size - shift_digits is always strictly positive.

The above doesn't depend on the precise values 55 and 30 - any other positive 
values would have worked, so even with 15-bit digits and some other double 
format with fewer bits, we still have "shift_digits < a_size".

And now since the v_rshift call writes "a_size - shift_digits" digits to x, 
we're guaranteed that at least one digit is written, so `x[0]` is not 
uninitialised.

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[issue40455] GCC 10 compiler warnings

[Mark Dickinson]

Mark Dickinson  added the comment:

I'm fairly sure that that's a false positive for `longobject.c`. Do you know of 
a non-intrusive way to silence the warning?

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[issue40455] GCC 10 compiler warnings

[Mark Dickinson]

Change by Mark Dickinson :


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nosy: +mark.dickinson

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[issue40455] GCC 10 compiler warnings

[STINNER Victor]

STINNER Victor  added the comment:

Andy Lester:
> Did you add any options to the ./configure call for cpython?  What were they?

I reported warnings that I saw on a buildbot build.

Extract of https://buildbot.python.org/all/#/builders/351/builds/406.

configure step:

./configure --prefix '$(PWD)/target' --with-lto

test.pythoninfo of the build says:

CC.version: gcc (GCC) 10.0.1 20200420 (Red Hat 10.0.1-0.12)

sysconfig[CCSHARED]: -fPIC
sysconfig[CC]: gcc -pthread
sysconfig[CFLAGS]: -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 
-Wall
sysconfig[CONFIG_ARGS]: '--prefix' 
'/home/buildbot/buildarea/3.x.cstratak-fedora-rawhide-ppc64le.lto/build/target' 
'--with-lto'
sysconfig[OPT]: -DNDEBUG -g -fwrapv -O3 -Wall
sysconfig[PY_CFLAGS]: -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 
-Wall
sysconfig[PY_CFLAGS_NODIST]: -flto -fuse-linker-plugin -ffat-lto-objects 
-flto-partition=none -g -std=c99 -Wextra -Wno-unused-result 
-Wno-unused-parameter -Wno-missing-field-initializers 
-Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal
sysconfig[PY_CORE_LDFLAGS]: -flto -fuse-linker-plugin -ffat-lto-objects 
-flto-partition=none -g
sysconfig[PY_LDFLAGS_NODIST]: -flto -fuse-linker-plugin -ffat-lto-objects 
-flto-partition=none -g
sysconfig[PY_STDMODULE_CFLAGS]: -Wno-unused-result -Wsign-compare -DNDEBUG -g 
-fwrapv -O3 -Wall -flto -fuse-linker-plugin -ffat-lto-objects 
-flto-partition=none -g -std=c99 -Wextra -Wno-unused-result 
-Wno-unused-parameter -Wno-missing-field-initializers 
-Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal 
-I. -I./Include

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[issue40455] GCC 10 compiler warnings

[Andy Lester]

Andy Lester  added the comment:

Did you add any options to the ./configure call for cpython?  What were they?

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[issue40455] GCC 10 compiler warnings

[Andy Lester]

Change by Andy Lester :


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nosy: +petdance

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[issue40455] GCC 10 compiler warnings

[STINNER Victor]

New submission from STINNER Victor :

GCC 10.0.1 on PPC64LE Fedora Rawhide LTO 3.x buildbot:
https://buildbot.python.org/all/#/builders/351/builds/406

Objects/longobject.c: In function ‘_PyLong_Frexp’:
Objects/longobject.c:2928:33: warning: ‘x_digits[0]’ may be used uninitialized 
in this function [-Wmaybe-uninitialized]
 2928 | x_digits[0] |= 1;
  | ^~

In function ‘assemble_lnotab’,
inlined from ‘assemble_emit’ at Python/compile.c:5709:25,
inlined from ‘assemble’ at Python/compile.c:6048:18:
Python/compile.c:5663:19: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]
 5663 | *lnotab++ = k;
  | ~~^~~

--
components: Build
messages: 367785
nosy: vstinner
priority: normal
severity: normal
status: open
title: GCC 10 compiler warnings
versions: Python 3.9

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