On 2025-03-03 18:00, Richard Earnshaw wrote:
On 28/02/2025 16:18, Richard Earnshaw wrote:
On 28/02/2025 16:12, Richard Earnshaw wrote:
On 08/11/2024 18:47, Torbjörn SVENSSON wrote:
Ok for trunk and releases/gcc-14?
--
A long time ago, this test forced -march=armv6.
With -marm, the generated assembler is:
foo:
sub r0, r0, #48
cmp r0, #9
movhi r0, #0
movls r0, #1
bx lr
With -mthumb, the generated assembler is:
foo:
subs r0, r0, #48
movs r2, #9
uxtb r3, r0
movs r0, #0
cmp r2, r3
adcs r0, r0, r0
uxtb r0, r0
bx lr
Looking at this code, I think both the uxtb instructions are unnecessary.
For the first UXTB. On entry, 'c' is a char (unsigned) so the value is passed
by the caller in a 32-bit reg, but range-limited (by the ABI) to values between
0 and 255.
We subtract 48 from that, so the range now, is from -48 to 207, but we're going
to look at this as an unsigned value, so it's effectively 0..207 or
UINT_MAX-48..UINT_MAX.
The UXTB instruction then converts the range so that the range becomes 0..255
again (but importantly, the values between UINT_MAX-48 and UINT_MAX are mapped to
the range 208..255. We then do an unsigned comparison between that value and the
constant 9 to test whether the result is >= 9.
Now, importantly, all the values that are in the range 208..255 are >= 9, but were
also >= 9 before the UXTB instruction. In effect, the UXTB is completely
redundant.
I said, >= 9 above, but it should be > 9 (hence condition code 'hi'). That does
affect the rest of the argument.
R.
The second UXTB is even more egregious. We have 0 in r0 and we then
conditionally add 1 to it, so r0 is in the range 0..1. Zero extending that
with a UXTB is therefore clearly pointless.
So neither of the UXTB instructions should be present, even if generating
Thumb1 code.
I think the failures are, therefore, real and we should look to fix the
compiler rather than 'fix' the scope of the test.
R.
Require effective-target arm_arm_ok to skip the test for thumb-only
targets (Cortex-M).
gcc/testsuite/ChangeLog:
* gcc.target/arm/unsigned-extend-1.c: Use effective-target
arm_arm_ok.
Signed-off-by: Torbjörn SVENSSON <torbjorn.svens...@foss.st.com>
---
gcc/testsuite/gcc.target/arm/unsigned-extend-1.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/gcc/testsuite/gcc.target/arm/unsigned-extend-1.c b/gcc/
testsuite/gcc.target/arm/unsigned-extend-1.c
index 3b4ab048fb0..73f2e1a556d 100644
--- a/gcc/testsuite/gcc.target/arm/unsigned-extend-1.c
+++ b/gcc/testsuite/gcc.target/arm/unsigned-extend-1.c
@@ -1,4 +1,5 @@
/* { dg-do compile } */
+/* { dg-require-effective-target arm_arm_ok } */
/* { dg-options "-O2" } */
unsigned char foo (unsigned char c)
I've just pushed a patch to eliminate one of the redundant zero-extends. The
other is harder to eliminate, so I've adjusted the test to xfail when compiling
for thumb1.
Hi Richard,
Can this be backported to releases/gcc-14 too?
I can do the backport if you just approve it.
Kind regards,
Torbjörn
