http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56546
Bug #: 56546
Summary: Using the divide operator on unsigned int produces
incorrect code on AVR
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: k...@free.fr
Created attachment 29592
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29592
Sample code to reproduce the issue
Using the divide integer on unsigned int variables on an AVR target leads to
wrong code being generated. The generated code uses the __umulhisi3 routine
from libgcc which is a multiplication routine and the result is always zero.
As I'm willing to increase my knowledge about the inner workings of gcc, I
tried to start debugging it myself. I found (using -fdump-rtl-all) that the
issue was occurring in the RTL part of the compiler where the udiv was at
several point transformed into a mult. I tried disabling the first pass that
did the transformation (-fdisable-rtl-fwprop1) and the udiv was propagated a
little further in the process until the ira pass.
I then tried to disable the ira pass using -fdisable-rtl-ira, made gcc crash
and discovered it was a really stupid thing to do
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56514).
So here I am again but his time leaving compiler debugging to the compiler guys
:-)
I've come up with a very simple source file that allows to reproduce the issue.
By the way I couldn't think of any case where transforming a udiv into a mult
operating on integers would make sense and would be glad if someone could give
me some hints on this.
Here's the command line I used:
avr-gcc -O0 -g -Wall -Wextra -save-temps -mmcu=atmega8 -o main.elf main.c
And the version of the toolchain components:
binutils: efb7cff2df30eb792d30e8afc384aa88c193932b
gcc: ef11013858b41453c4953ca8d4c25e3b1668e536
avr-libc: 2ac01d285e23894ef3bcc65c75b39da8157b9fd9
gcc-4.7.2, binutils 2.23.1 and avr-libc 1.8.0 give the same result.
