http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49543
Summary: Cast move causes incorrect code and numerical results
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: acarm...@mathworks.com
Created attachment 24606
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24606
Folded and unfolded expressions calculate different results
Overview
--------
In the following example, "Data" is a 32-bit int, and "tmp" is a 64-bit int.
with gcc -ansi -pedantic the following line:
tmp = (int64_T)(Data - 1) * 1805715409L; /* 32-bit subtraction */
is compiled as
tmp = ((int64_T)Data - 1) * 1805715409L; /* 64-bit subtraction */
This changes the numerical result if Data-1 overflowed.
If the code was not folded, as in this example:
int32_T hoist;
hoist = Data - 1;
tmp = (int64_T)hoist * 1805715409L;
the result in tmp is computed correctly.
The attached file cast_move_bug.c demonstrates this and has copiling
instructions.
The attached file cast_move_bug.s shows that the location of the up-cast to 64
with cltq instruction moved to *before* the subtraction (compared to the
unfolded version).
Triggering
----------
The bug was observed on all versions of gcc up to 4.6.0 on 64-bit Linux.
When -pedantic *IS NOT* specified, the bug is triggered only with -O0. Under
-O1 and above the bug is not triggered.
When -pedantic *IS* specified, the bug is triggered under all optimization
levels: -O0 through -O3.
g++ 4.3.2 and 4.4.3 do not exhibit this bug.
Claim
-----
The up-cast to 64-bit was incorrectly moved inside the parentheses, violating
the effect of parenthesization on type sizes, and causing the subtraction to
become 64-bit.
Fix request
-----------
MathWorks requests the fix to be back-ported to 4.5 - a reasonably current, and
stable release within our customer community.
