[Bug target/48576] wrong code when accessing variables in a large stack frame

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48576

--- Comment #5 from Mikael Pettersson <mikpe at it dot uu.se> 2011-04-16 19:28:54 UTC ---
It started with r146817:

Author: matz
Date: Sun Apr 26 19:35:04 2009
New Revision: 146817

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=146817
Log:
gcc/
        Expand from SSA.
        ...

Diffing the assembly code from r146816 and r146817 shows the error:

--- pr48576.s-r146816   2011-04-16 21:06:24.000000000 +0200
+++ pr48576.s-r146817   2011-04-16 21:06:03.000000000 +0200
...
        bl      my_realloc
        sub     r3, fp, #4096
-       str     r0, [r3, #-44]
-       str     r6, [r0, r5, asl #2]
-       cmp     r4, #1
+       str     r0, [r3, #-48]
+       ldr     r3, [r3, #-44]
+       ldr     r2, [r3, #-48]
+       str     r4, [r2, r3, asl #2]
+       cmp     r9, #1
        ble     .L16
...

We wish to save the return value of my_realloc() and immediately make a store
with it as an array base pointer and another value as index.  Starting with
r146817 both values are considered spilled and get reloaded.  Both reloads need
an intermediate pointer into the frame to reach their locations.  The reload of
the index value clobbers the intermediate frame pointer needed for the reload
of the array base, causing that reload to read an incorrect location.