This test simulates the process of clearing the "present" bit
in an x86 page table entry. The code *should* load the PTE,
clear the present bit, and store the PTE. On x86, it is
possible to perform these 3 steps in a single instruction, i.e.
AND memory with immediate operand. The code should also be
preceede with a null check for the access object.
package Bit_Test is
type Page_Frame_Number is
mod 2 ** 20;
type Page_Table_Entry is
record
P : Boolean;
RW : Boolean;
U : Boolean;
PWT : Boolean;
PCD : Boolean;
A : Boolean;
D : Boolean;
PSE : Boolean;
G : Boolean;
PFN : Page_Frame_Number;
end record;
for Page_Table_Entry use
record
P at 0 range 0 .. 0;
RW at 0 range 1 .. 1;
U at 0 range 2 .. 2;
PWT at 0 range 3 .. 3;
PCD at 0 range 4 .. 4;
A at 0 range 5 .. 5;
D at 0 range 6 .. 6;
PSE at 0 range 7 .. 7;
G at 0 range 8 .. 8;
PFN at 0 range 12 .. 31;
end record;
type Page_Table_Entry_Access is
access Page_Table_Entry;
procedure Invalidate_PTE (
PTE : in Page_Table_Entry_Access
);
end Bit_Test;
package body Bit_Test is
procedure Invalidate_PTE (
PTE : in Page_Table_Entry_Access
) is
begin
PTE.all := (
P => False,
RW => PTE.RW,
U => PTE.U,
PWT => PTE.PWT,
PCD => PTE.PCD,
A => PTE.A,
D => PTE.D,
PSE => PTE.PSE,
G => PTE.G,
PFN => PTE.PFN
);
end Invalidate_PTE;
end Bit_Test;
Code generated for Invalidate_PTR:
000000a0 <bit_test__invalidate_pte>:
a0: 55 push %ebp
a1: 89 e5 mov %esp,%ebp
a3: 57 push %edi
a4: 56 push %esi
a5: 53 push %ebx
a6: 83 ec 1c sub $0x1c,%esp
a9: 8b 7d 08 mov 0x8(%ebp),%edi
ac: 85 ff test %edi,%edi
ae: 0f 84 e0 00 00 00 je 194 <bit_test__invalidate_pte+0xf4>
b4: 0f b6 07 movzbl (%edi),%eax
b7: 88 c2 mov %al,%dl
b9: 88 c1 mov %al,%cl
bb: d0 ea shr %dl
bd: 88 c3 mov %al,%bl
bf: 80 e2 01 and $0x1,%dl
c2: 88 55 eb mov %dl,0xffffffeb(%ebp)
c5: 88 c2 mov %al,%dl
c7: c0 ea 04 shr $0x4,%dl
ca: 80 e2 01 and $0x1,%dl
cd: 88 55 ef mov %dl,0xffffffef(%ebp)
d0: 88 c2 mov %al,%dl
d2: c0 ea 05 shr $0x5,%dl
d5: 80 e2 01 and $0x1,%dl
d8: 88 55 f0 mov %dl,0xfffffff0(%ebp)
db: 88 c2 mov %al,%dl
dd: c0 ea 06 shr $0x6,%dl
e0: 80 e2 01 and $0x1,%dl
e3: 88 55 f1 mov %dl,0xfffffff1(%ebp)
e6: 88 c2 mov %al,%dl
e8: 24 fe and $0xfe,%al
ea: c0 ea 07 shr $0x7,%dl
ed: 88 55 f2 mov %dl,0xfffffff2(%ebp)
f0: c0 e9 02 shr $0x2,%cl
f3: 0f b6 57 01 movzbl 0x1(%edi),%edx
f7: 80 e1 01 and $0x1,%cl
fa: c0 eb 03 shr $0x3,%bl
fd: 80 e3 01 and $0x1,%bl
100: 80 e2 01 and $0x1,%dl
103: 88 55 f3 mov %dl,0xfffffff3(%ebp)
106: 8b 37 mov (%edi),%esi
108: 88 07 mov %al,(%edi)
10a: 0f b6 55 eb movzbl 0xffffffeb(%ebp),%edx
10e: 8b 07 mov (%edi),%eax
110: 01 d2 add %edx,%edx
112: 83 e0 fd and $0xfffffffd,%eax
115: 09 d0 or %edx,%eax
117: 0f b6 d1 movzbl %cl,%edx
11a: 89 07 mov %eax,(%edi)
11c: c1 e2 02 shl $0x2,%edx
11f: 83 e0 fb and $0xfffffffb,%eax
122: 09 d0 or %edx,%eax
124: 0f b6 d3 movzbl %bl,%edx
127: 89 07 mov %eax,(%edi)
129: c1 e2 03 shl $0x3,%edx
12c: 83 e0 f7 and $0xfffffff7,%eax
12f: 09 d0 or %edx,%eax
131: 89 07 mov %eax,(%edi)
133: 83 e0 ef and $0xffffffef,%eax
136: 0f b6 55 ef movzbl 0xffffffef(%ebp),%edx
13a: c1 e2 04 shl $0x4,%edx
13d: 09 d0 or %edx,%eax
13f: 89 07 mov %eax,(%edi)
141: 83 e0 df and $0xffffffdf,%eax
144: 0f b6 55 f0 movzbl 0xfffffff0(%ebp),%edx
148: c1 e2 05 shl $0x5,%edx
14b: 09 d0 or %edx,%eax
14d: 89 07 mov %eax,(%edi)
14b: 09 d0 or %edx,%eax
14d: 89 07 mov %eax,(%edi)
14f: 83 e0 bf and $0xffffffbf,%eax
152: 0f b6 55 f1 movzbl 0xfffffff1(%ebp),%edx
156: c1 e2 06 shl $0x6,%edx
159: 09 d0 or %edx,%eax
15b: 89 07 mov %eax,(%edi)
15d: 0f b6 55 f2 movzbl 0xfffffff2(%ebp),%edx
161: c1 e2 07 shl $0x7,%edx
164: 25 7f ff ff ff and $0xffffff7f,%eax
169: 09 d0 or %edx,%eax
16b: 81 e6 00 f0 ff ff and $0xfffff000,%esi
171: 89 07 mov %eax,(%edi)
173: 25 ff fe ff ff and $0xfffffeff,%eax
178: 0f b6 55 f3 movzbl 0xfffffff3(%ebp),%edx
17c: c1 e2 08 shl $0x8,%edx
17f: 09 d0 or %edx,%eax
181: 89 07 mov %eax,(%edi)
183: 25 ff 0f 00 00 and $0xfff,%eax
188: 09 f0 or %esi,%eax
18a: 89 07 mov %eax,(%edi)
18c: 83 c4 1c add $0x1c,%esp
18f: 5b pop %ebx
190: 5e pop %esi
191: 5f pop %edi
192: 5d pop %ebp
193: c3 ret
--
Summary: Poor bit-field code generation
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ada
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dave at synergy dot org
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20671
