Checking out PR4483 (constant overflow on PPC, problems compiling Linux kernel)

[Corey Minyard]

I tried to compile the Linux kernel on PowerPC, and I ran into a problem 
someone previously reports (PR4483, 
http://gcc.gnu.org/ml/gcc-prs/2001-10/msg00105.html).  I've played 
around with it some, and it looks like a constant is overflowing in the 
gcse stage.  The PR has a testcase in it.

In the situation, an INSN references two other registers that are 
constants (one is 0xffffffff80000000, the other is 42) and subtracts the 
second from the first (MINUS code).  In validate_replace_rtx_1 
(recog.c), it replaces the registers with constants, then adds them. 
 This results in a constant 0xffffffff7fffffd6, which is obviously now 
too negative to fit into an SI register.  The original constant was 
positive (0x80000000UL in the source).  Since constants don't have 
modes, the addition code doesn't handle this (and it doesn't look like 
it cares, anyway).

I have attached a patch that adds a PowerPC production to truncate a 
constant that is too large that goes into a register.  However, I'm not 
sure if this is the right way to handle this, but it at least works 
around the problem.  The  main production that handles this won't work 
because it checks for constant overflows.

I compiled the Linux kernel (after working around a few small problems 
there) successfully.  However, it crashed at startup :-(.

-Corey
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: gcc-ppc-of-1.diff
URL: <https://gcc.gnu.org/pipermail/gcc/attachments/20011120/52de45be/attachment.ksh>