http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54398
Carrot <carrot at google dot com> changed:
What |Removed |Added
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CC| |carrot at google dot com
--- Comment #4 from Carrot <carrot at google dot com> 2012-09-07 01:19:31 UTC
---
The code before the position Ahmad pointed out is already wrong.
The fault instruction sequence is:
asrs r5, r5, #1
asr ip, ip, #1 ; A, tmp1.x
asrs r0, r0, #1 ; B, tmp1.y
asrs r6, r6, #1
mov r4, r1
add r8, ip, r6 ; C, tmp3.x
add r9, r0, r5 ; D, tmp3.y
add r7, sp, #0
asr r1, r8, #1
add ip, r4, #8 ; E,
asr r9, r9, #1
str r1, [r7, #16]
str r9, [r7, #20]
ldmia r3, {r0, r1} ; F,
stmia r4, {r0, r1}
Instruction A computes the result of tmp1.x, instruction C use it to compute
tmp3.x, instruction E overwrite the value of tmp1.x. But in the source code,
tmp1.x is still needed to execute "dst1->p2 = tmp1;", so at last dest1->p2.x
gets garbage.
Similarly instruction B computes tmp1.y, instruction D uses it to compute
tmp3.y, instruction F overwrites it. After executing "dst1->p2 = tmp1;",
dst1->p2.y gets another garbage value.
For comparison, following is the correct version
asrs r7, r7, #1 ; A, tmp1.x
asrs r0, r0, #1 ; B, tmp1.y
asrs r6, r6, #1
asrs r5, r5, #1
sub sp, sp, #28
mov r4, r1
add r8, r7, r6 ; C, tmp3.x
add ip, r0, r5 ; D, tmp3.y
str r7, [sp, #0] ; X, save tmp1.x
str r0, [sp, #4] ; Y, save tmp1.y
asr r1, ip, #1
add r7, r4, #8 ; E
asr r8, r8, #1
str r1, [sp, #20]
str r8, [sp, #16]
ldmia r3, {r0, r1} ; F
stmia r4, {r0, r1}
The obvious difference is the extra instructions X and Y, they save the value
of tmp1 to stack before reusing the register.
The simplified preprocessed source code is
struct A
{
int x;
int y;
void f(const A &a, const A &b)
{
x = (a.x + b.x)>>1;
y = (a.y + b.y)>>1;
}
};
class C {
public:
A p1;
A p2;
A p3;
bool b;
void g(C *, C *) const;
};
void C::g(C *dst1, C *dst2) const
{
A tmp1, tmp2, tmp3;
tmp1.f(p2,p1);
tmp2.f(p2,p3);
tmp3.f(tmp1, tmp2);
dst1->p1 = p1;
dst1->p2 = tmp1;
dst1->p3 =
dst2->p1 = tmp3;
dst2->p2 = tmp2;
dst2->p3 = p3;
}
The simplified command line is:
./cc1plus -fpreprocessed t.ii -quiet -dumpbase t.cpp -mthumb "-march=armv7-a"
"-mtune=cortex-a15" -auxbase t -O2 -fno-omit-frame-pointer -o t.s
It looks like the dse2 pass did wrong transformation.
The gcc4.7 and trunk generate correct code.
