I would like to add some explanations: This is the issue gone from GCC behavior on VLA allocation. I did a simple test with VLA, and the following snippet from its ASM listing may clarify the root cause of issue:
VLA allocation start. R1 is initialized by the length of VLA. >80080030: e281300f add r3, r1, #15 ; 0xf >80080034: e2033f7e and r3, r3, #504 ; 0x1f8 Align VLA size. >80080038: e1a0500d mov r5, sp Save SP to recover it when VLA becomes needless. >8008003c: e04dd003 sub sp, sp, r3 Allocate R3 bytes on stack. >80080040: e1a0300d mov r3, sp Store VLA address in R3. >80080044: e1a0c1a3 lsr ip, r3, #3 >80080048: e1a0218c lsl r2, ip, #3 Here VLA address is aligned by 8 bytes. If SP is either 0xYYYYYYY4 or 0xZZZZZZZC, r2 will lose significant digit and will become 0xYYYYYYY0/0xZZZZZZZ8 (VLA=SP-4). It will less than SP, so the next 'push' (alias to STMDB) will decrement SP by 4 and will store register at the top of the stack, so this will overwrite first 4 bytes of VLA. On 06/15/2010 10:18 PM, Vitaly Kuzmichev wrote: > The ARM ABI requires that the stack be aligned to 8 bytes as it is noted > in Procedure Call Standard for the ARM Architecture: > http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/index.html > > Unaligned SP also causes the problem with variable-length arrays > allocation when VLA address becomes less than stack pointer during > aligning of this address, so the next 'push' in the stack overwrites > first 4 bytes of VLA. > > Signed-off-by: Vitaly Kuzmichev <[email protected]> _______________________________________________ U-Boot mailing list [email protected] http://lists.denx.de/mailman/listinfo/u-boot
