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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - ARM struct byval size > 64 triggers failure"
href="http://llvm.org/bugs/show_bug.cgi?id=16368">16368</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>ARM struct byval size > 64 triggers failure
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: ARM
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>viswabramana.rajesh@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Handling of pass byval of struct size >64 bytes case is going wrong for arm
targets.
Test Code:
#include <stdio.h>
struct S114 {
char a[67];
}a114[5];
struct S114 check114 (struct S114 arg0, struct S114* arg1) {
if(&a114[0] != arg1) // arg1 value is wrong
printf( "values %p, %p\n", &a114[0], arg1);
}
int main () {
int i= 0, j = 0;
for (;j<2; j++) //filling a114 with some values for mark
for(i=0; i<sizeof(struct S114); i++)
memset(&a114[j].a[i],(0x11+i+j*30), sizeof(int));
check114 (a114[1], &a114[0]);
}
Output:
# ./check114.exe
values 0x10861, 0x4071706f --> which is wrong.
Assembly for check114
---------------------------------------------------------------
sub sp, sp, #16
push {r11, lr}
mov r11, sp
sub sp, sp, #8
str r3, [r11, #20]
str r2, [r11, #16]
str r1, [r11, #12]
ldr r1, [r11, #76]
str r1, [sp, #4]
.loc 1 7 0 prologue_end
ldr r2, .LCPI0_0
cmp r2, r1
beq .LBB0_2
b .LBB0_1
---------------------------------------------------------------
>From reg, stack dump:
---------------------------------------------------------------
@entry of check114:
=> 0x8398 <check114>: sub sp, sp, #16
sp 0xbefff808 0xbefff808
@if condition :
0x83b4 <check114+28>: ldr r1, [r11, #76] ; 0x4c <- wrong value copied
offset #76 should be #80
0x83b8 <check114+32>: str r1, [sp, #4]
0x83bc <check114+36>: ldr r2, [pc, #44] ; 0x83f0 <check114+88>
=> 0x83c0 <check114+40>: cmp r2, r1
r11 0xbefff7f0 -1090521104
sp 0xbefff7e8 0xbefff7e8
Stack dump:
0xbefff7e4: 0x4001ed08 0x40024f90 0x4071706f 0xbefff8a0
0xbefff7f4: 0x0000869c 0x00000000 0x3231302f 0x36353433
0xbefff804: 0x3a393837 0x3e3d3c3b 0x4241403f 0x46454443
0xbefff814: 0x4a494847 0x4e4d4c4b 0x5251504f 0x56555453
0xbefff824: 0x5a595857 0x5e5d5c5b 0x6261605f 0x66656463
0xbefff834: 0x6a696867 0x6e6d6c6b 0x4071706f 0x00010861 //[R11+4c] ---->
[0xbefff7f0+4c] ---> [0xbefff83c] ----> 0x4071706f
>From ARM Lowering part
ARMTargetLowering::computeRegArea(..) {
...
VARegSize = NumGPRs * 4;
VARegSaveSize = (VARegSize + Align - 1) & ~(Align - 1);//-> 8 byte alignment
done here
}
For above test code case,
NumGPRs = 3 registers (used to hold struct byval) = 3*4
VARegSaveSize = 16 (after considering 8 byte alignment )
So, 4 bytes offset added due to alignment.
Which results,
sub sp, sp, #16
- Offset(#76) for the instruction, "ldr r1, [r11, #76] ; 0x4c" when
calculated, 4 bytes alignment is considered.
- Prologue stackpointer calculation 8 byte alignment is considered.
Due to this alignment mismatch, when try to access second or rest of arguments
after struct byval goes wrong.
- Issue occurs for only if odd number of registers used for struct byval.
- If even number of registers used for byval, issue doesnt occur(since it will
be naturally aligned by 8 byte boundary when copied to stack).
Ex: Update of above test case with int,
struct S114 check114 (int a, struct S114 arg0, struct S114* arg1) { //
accessing arg1 is fine in this case
.....
} //works fine.</pre>
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