MIPS Wrong-code regression.

Wed Jan 31 18:36:00 GMT 2007

Andrew Haley wrote:
> David Daney writes:
> > Richard,
> > 
> > Sometime between 1/7 and 1/16 on the trunk I started getting wrong code 
> > on a bunch of java testcases under mipsel-linux.
> > 
> > It looks related to (but not necessarily caused by) this patch:
> > 
> > http://gcc.gnu.org/ml/gcc-patches/2006-03/msg01346.html
> > 
> > For example if we examine the assembler output of the PR9577.java 
> > testcase, we see:
> > 
> > .
> > .
> > .
> > $LBB2:
> > lw 2,40ドル($fp)
> > sw 2,24ドル($fp)
> > lw 2,24ドル($fp)
> > move 4,ドル2ドル
> > .option pic0
> > jal _ZN4java4lang6ObjectC1Ev
> > nop
> > 
> > .option pic2
> > lw 28,16ドル($fp)
> > $LBE2:
> > move $sp,$fp
> > lw 31,36ドル($sp)
> > lw $fp,32($sp)
> > addiu $sp,$sp,40
> > j 31ドル
> > nop
> > 
> > The call to _ZN4java4lang6ObjectC1Ev is being generated as non-pic, even 
> > though that symbol is defined in libgcj.so. The assembler and linker 
> > conspire to jump to address 0x00000000 for this call.
> > 
> > It looks like the logic that decides if a symbol is external to the 
> > compilation unit is faulty.
> > 
> > Any ideas about where it might have gone wrong?
>> Does http://gcc.gnu.org/ml/gcc/2007-01/msg01184.html fix this?
>
Unfortunately no. The following output is generated with r121186 + 
Andrew.s patched class.c
There are several problems with the generated code for the failing class:
public class PR9577
{
 private native void sayHello (String[] s, Object o);
 public static void main (String[] args)
 {
 PR9577 x = new PR9577( );
 x.sayHello( null, null);
 }
}
Note that this class has an implicit public constructor that does 
nothing other than call the super class (java.lang.Object) constructor.
/home/build/gcc-build/gcc/gcj -v 
-B/home/build/gcc-build/mipsel-unknown-linux-gnu/libjava/ 
-B/home/build/gcc-build/gcc/ --encoding=UTF-8 
-B/home/build/gcc-build/mipsel-unknown-linux-gnu/libjava/testsuite/../ 
/home/build/gcc/libjava/testsuite/libjava.cni/PR9577.jar -o j.s -S
Here is the entire generated code for the constructor:
 .globl _ZN6PR9577C1Ev
 .ent _ZN6PR9577C1Ev
 .type _ZN6PR9577C1Ev, @function
_ZN6PR9577C1Ev:
$LFB2:
 .frame $fp,40,31ドル # vars= 8, regs= 2/0, args= 16, 
gp= 8
 .mask 0xc0000000,-4
 .fmask 0x00000000,0
 .set noreorder
 .set nomacro
 addiu $sp,$sp,-40
$LCFI0:
 sw 31,36ドル($sp)
$LCFI1:
 sw $fp,32($sp)
$LCFI2:
 move $fp,$sp
$LCFI3:
 .cprestore 16
 sw 4,40ドル($fp)
$LBB2:
 lw 2,40ドル($fp)
 sw 2,24ドル($fp)
 lw 2,24ドル($fp)
 move 4,ドル2ドル
 .option pic0
 jal _ZN4java4lang6ObjectC1Ev
 nop
 .option pic2
 lw 28,16ドル($fp)
$LBE2:
 move $sp,$fp
 lw 31,36ドル($sp)
 lw $fp,32($sp)
 addiu $sp,$sp,40
 j 31ドル
 nop
 .set macro
 .set reorder
$LFE2:
 .end _ZN6PR9577C1Ev
Here are the problems I see:
1) The call to _ZN4java4lang6ObjectC1Ev is absolute instead of via the 
plt. That function is in a shared library not this compilation unit.
2) It is a public global method. $gp should be initialized, but it is not.
If I compile it with -O3 -mshared I get:
 .globl _ZN6PR9577C1Ev
 .ent _ZN6PR9577C1Ev
 .type _ZN6PR9577C1Ev, @function
_ZN6PR9577C1Ev:
$LFB2:
 .frame $sp,32,31ドル # vars= 0, regs= 1/0, args= 16, 
gp= 8
 .mask 0x80000000,-8
 .fmask 0x00000000,0
 .set noreorder
 .cpload 25ドル
 .set nomacro
 addiu $sp,$sp,-32
$LCFI0:
 sw 31,24ドル($sp)
$LCFI1:
 .cprestore 16
 lw 25,ドル%call16(_ZN4java4lang6ObjectC1Ev)(28ドル)
 jalr 25ドル
 nop
 lw 28,16ドル($sp)
 lw 31,24ドル($sp)
 j 31ドル
 addiu $sp,$sp,32
 .set macro
 .set reorder
$LFE2:
 .end _ZN6PR9577C1Ev
This time the call to _ZN4java4lang6ObjectC1Ev *is* done via the plt, 
but we are using .cpload instead of having gcc generate the individual 
instructions and interleaving them with the $sp adjustment as it 
normally does.
David Daney