Re: [Qemu-ppc] [Qemu-devel] [PATCH 00/34] tcg, target/ppc vector improvements

[Mark Cave-Ayland]

On 18/12/2018 09:49, Mark Cave-Ayland wrote:

> Following on from this, the next patch "target/ppc: convert vsplt[bhw] to use 
> vector
> operations" causes corruption of the OS X splash screen
> (https://www.ilande.co.uk/tmp/qemu/badapple2.png) in a way that suggests 
> there may be
> an endian issue.

After some more digging I've found out what's going on here by dumping out the 
AVR
registers before and after:

Before the patch:

BEFORE:
uimm: 0  size: 2
sreg: 99 @ 0x7f54fd7157a0 - 1 6a 1 d9 1 15 fd 63 0 0 0 0 0 0 0 0
dreg: 99 @ 0x7f54fd715870 - 7f ff de ad 7f ff de ad 7f ff de ad 7f ff de ad
AFTER:
dreg: 99 @ 0x7f54fd715870 - 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a

BEFORE:
uimm: 1  size: 2
sreg: 99 @ 0x7f54fd7157a0 - 1 6a 1 d9 1 15 fd 63 0 0 0 0 0 0 0 0
dreg: 99 @ 0x7f54fd715870 - 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a
AFTER:
dreg: 99 @ 0x7f54fd715870 - 1 d9 1 d9 1 d9 1 d9 1 d9 1 d9 1 d9 1 d9


After the patch:

BEFORE:
uimm: 0  size: 2
sreg: 5 @ 0x7fe5a0c4a7a0 - 1 6a 1 d9 1 15 fd 63 0 0 0 0 0 0 0 0
dreg: 18 @ 0x7fe5a0c4a870 - 7f ff de ad 7f ff de ad 7f ff de ad 7f ff de ad
AFTER:
dreg: 18 @ 0x7fe5a0c4a870 - 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1

BEFORE:
uimm: 1  size: 2
sreg: 5 @ 0x7fe5a0c4a7a0 - 1 6a 1 d9 1 15 fd 63 0 0 0 0 0 0 0 0
dreg: 18 @ 0x7fe5a0c4a870 - 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1 5d 1
AFTER:
dreg: 18 @ 0x7fe5a0c4a870 - 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1 6a 1


As you can see vsplth splat is one byte off with this patch applied and the 
cause is
the xor in the #ifndef HOST_WORDS_BIGENDIAN block: before the xor is applied, 
bofs is
aligned to 2 bytes and with bofs ^ 15 the LSB is set to 1 again, introducing 
the 1
byte error.

Applying the following patch to mask bofs based upon the size of vece seems to 
fix
the issue here for me on little-endian Intel:

diff --git a/target/ppc/translate/vmx-impl.inc.c 
b/target/ppc/translate/vmx-impl.inc.c
index 59d3bc6e02..41ddbd879f 100644
--- a/target/ppc/translate/vmx-impl.inc.c
+++ b/target/ppc/translate/vmx-impl.inc.c
@@ -815,6 +815,7 @@ static void gen_vsplt(DisasContext *ctx, int vece)
     bofs += (uimm << vece) & 15;
 #ifndef HOST_WORDS_BIGENDIAN
     bofs ^= 15;
+    bofs &= ~((1 << vece) - 1);
 #endif

     tcg_gen_gvec_dup_mem(vece, dofs, bofs, 16, 16);


ATB,

Mark.