[Bug target/23774] New: dealloc of dynamic stack space breaks backchain

[amodra at bigpond dot net dot au]

The following compiled with -m32 -O2 -S

void badFunc (int size)
{ 
  char temp[size];
  temp[size-1] = '\0';
};

gives

badFunc:
        mflr 0
        stwu 1,-16(1)
        stw 0,20(1)
        addi 0,3,30
        lwz 9,0(1)
        mr 11,1
        stw 31,12(1)
        mr 31,1
        rlwinm 0,0,0,0,27
        neg 0,0
        stwux 9,1,0
        li 9,0
        addi 0,1,23
        rlwinm 0,0,0,0,27
        add 3,3,0
        stb 9,-1(3)      <- old backchain possibly overwritten
        nop
        nop
        nop
        lwz 0,0(1)
        mr 1,11          <- adjust stack, backchain possibly invalid!
        stw 0,0(1)       <- write backchain
        nop
        lwz 11,0(1)
        lwz 0,4(11)
        lwz 31,-4(11)
        mr 1,11
        mtlr 0
        blr

This testcase also shows
a) excess allocation of dynamic stack space,
b) needless alignment of dynamic stack space,
c) poor epilogue code, with unnecesary stack adjustments.

-- 
           Summary: dealloc of dynamic stack space breaks backchain
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: amodra at bigpond dot net dot au
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: powerpc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23774