[PATCH v2] Upgrade arm-sflt minimum architecture to ARMv5TE

[Jakub Vaněk]

Hi David,

On 2018-11-26 at 17:36 +1000, David Holmes wrote:
> Hi Jakub,
> 
> On 26/11/2018 1:53 am, Jakub Vaněk wrote:
> > Hi,
> > 
> > This patch raises the minimum architectural level for ARM CPUs to
> > ARMv5TE. It is done through changing the -march flag in the current
> > CPU-specific CFLAGS and ASFLAGS.
> > 
> > This patch depends on "Append assembler flags on ARM targets" patch
> > for
> > ASFLAGS handling.
> > 
> > Reason for this change: assembler code in linux_arm_32.s uses PLD
> > instructions. ARM ISA manual mentions this: "This instruction is
> > available in E variants of ARM architecture v5 and above." (thanks
> > to
> > David Holmes for noticing this).
> 
> FYI I did a bit of digging and we tried to make this change back in
> 2010 
> and ran into problems:
> 
> "It appears that gcc assumes that if you choose the armv5te
> architecture 
> option, it can use the "E" instruction extensions which provide 64
> bit 
> load and stores to 4 byte aligned addresses. I tried two different 
> compiler versions 4.1.2 and 4.2.3 and they both had the same
> problem. 
> The <hardware> does support ldrd/strd but they do not support 4 byte 
> aligned operations. The addresses must be 8 byte aligned."

> How things stand today I can not say.

I have searched for this and indeed, this bug wasn't fixed until
recently: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82445

For our own use, it seems that Ubuntu gcc7 is patched, whether Debian
gcc6 isn't. I think that the best solution is to keep armv5t in CFLAGS
and only raise armv5te in ASFLAGS, where it is actually needed. This
way GCC still generates older non-buggy code and the assembler works
with pld. I have tested this configuration and it compiles
successfully.


https://ci.adoptopenjdk.net/view/ev3dev/job/openjdk12_build_ev3_linux/40/

Thanks,

Jakub

> 
> Cheers,
> David

# HG changeset patch
# User Jakub Vaněk <linuxtardis at gmail.com>
# Date 1543252130 -3600
#      Mon Nov 26 18:08:50 2018 +0100
# Node ID bfd9e7032cfdc8c814ddaf4bb471a014fbbf1a89
# Parent  d2db3b32d5614029f2b1af2731c0d9ec10d88767
Fix undefined pld instruction on arm-sflt

diff --git a/make/autoconf/flags.m4 b/make/autoconf/flags.m4
--- a/make/autoconf/flags.m4
+++ b/make/autoconf/flags.m4
@@ -46,6 +46,11 @@
     AC_MSG_CHECKING([for ABI profle])
     AC_MSG_RESULT([$OPENJDK_TARGET_ABI_PROFILE])
 
+    # --- Arm-sflt CFLAGS and ASFLAGS ---
+    # Armv5te is required for assembler, because pld insn used in arm32 hotspot is only in v5E and above.
+    # However, there is also a GCC bug which generates unaligned strd/ldrd instructions on armv5te:
+    # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82445, and it was fixed only quite recently.
+    # The resulting compromise is to enable v5TE for assembler and let GCC generate code for v5T.
     if test "x$OPENJDK_TARGET_ABI_PROFILE" = xarm-vfp-sflt; then
       ARM_FLOAT_TYPE=vfp-sflt
       ARM_ARCH_TYPE_FLAGS='-march=armv7-a -mthumb'
@@ -57,11 +62,11 @@
     elif test "x$OPENJDK_TARGET_ABI_PROFILE" = xarm-sflt; then
       ARM_FLOAT_TYPE=sflt
       ARM_ARCH_TYPE_FLAGS='-march=armv5t -marm'
-      ARM_ARCH_TYPE_ASFLAGS='-march=armv5t'
+      ARM_ARCH_TYPE_ASFLAGS='-march=armv5te'
     elif test "x$OPENJDK_TARGET_ABI_PROFILE" = xarmv5-vfp-sflt; then
       ARM_FLOAT_TYPE=vfp-sflt
       ARM_ARCH_TYPE_FLAGS='-march=armv5t -marm'
-      ARM_ARCH_TYPE_ASFLAGS='-march=armv5t'
+      ARM_ARCH_TYPE_ASFLAGS='-march=armv5te'
     elif test "x$OPENJDK_TARGET_ABI_PROFILE" = xarmv6-vfp-hflt; then
       ARM_FLOAT_TYPE=vfp-hflt
       ARM_ARCH_TYPE_FLAGS='-march=armv6 -marm'