On Fri, 18 Nov 2016, Janne Grunau wrote:
On 2016-11-18 10:06:38 +0200, Martin Storsjö wrote:
This allows using the cbz/cbnz instruction in thumb mode, for the
cases where the branch target is close enough.
---
libavutil/arm/asm.S | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/libavutil/arm/asm.S b/libavutil/arm/asm.S
index 4ac0ea2..e66c34c 100644
--- a/libavutil/arm/asm.S
+++ b/libavutil/arm/asm.S
@@ -326,6 +326,18 @@ T strh \rt, [\rn]
T sub \rn, \rn, \rm
.endm
+#if !CONFIG_THUMB
+.macro cbz rd, target
+ cmp \rd, #0
+ beq \target
+.endm
+
+.macro cbnz rd, target
+ cmp \rd, #0
+ bne \target
+.endm
+#endif
I'm not convinced this is a good idea, the thumb instruction works only
with r0-r7 and has a very limited range while the arm version has no
such limitations. I.e. unless one tests thumb explicitly it's very easy
to break thumb. For the few places where it is useful 'T' and 'A'
prefixed instructions are imo the better way. Using instructions setting
APSR flags are preferable to cbn?z. This differs on aarch64 since there
only a some instructions can set flags.
Also I optimized one intended use away.
Fair enough. Yes, the limited range makes it impractical in many cases, I
had expected to use it in the vp9 loop filter.
I also noticed now that gas-preprocessor doesn't handle cbz/cbnz
translation of branch targets (1f etc) for armasm yet, we apparently don't
use it anywhere yet.
Patch dropped then.
// Martin
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