Gentoo ARM64 Altra Compile chromium Issue

When compile the chromium on my Altra platform, it meet compile problem like this

no-dangling-assignment-gsl -std=c++20 -Wno-trigraphs -gsimple-template-names -fno-exceptio00:01:51 [472/19443]
nc++ -isystem../../third_party/libc++/src/include -isystem../../third_party/libc++abi/src/include -fvisibility
-inlines-hidden -O2 -pipe -march=armv8.2-a -Wno-unknown-warning-option -c ../../third_party/libyuv/source/comp
are_neon64.cc -o obj/third_party/libyuv/libyuv_neon/compare_neon64.o
../../third_party/libyuv/source/compare_neon64.cc:175:8: error: instruction requires: dotprod
  175 |       "udot        v4.4s, v0.16b, v6.16b         \n"
      |        ^
<inline asm>:14:1: note: instantiated into assembly here
   14 | udot        v4.4s, v0.16b, v6.16b
      | ^
../../third_party/libyuv/source/compare_neon64.cc:176:8: error: instruction requires: dotprod
  176 |       "udot        v5.4s, v1.16b, v6.16b         \n"
      |        ^
<inline asm>:15:1: note: instantiated into assembly here
   15 | udot        v5.4s, v1.16b, v6.16b
      | ^
../../third_party/libyuv/source/compare_neon64.cc:204:8: error: instruction requires: dotprod
  204 |       "udot        v4.4s, v0.16b, v0.16b         \n"
      |        ^
<inline asm>:10:1: note: instantiated into assembly here
   10 | udot        v4.4s, v0.16b, v0.16b
      | ^
../../third_party/libyuv/source/compare_neon64.cc:205:8: error: instruction requires: dotprod
  205 |       "udot        v5.4s, v1.16b, v1.16b         \n"
      |        ^
<inline asm>:11:1: note: instantiated into assembly here
   11 | udot        v5.4s, v1.16b, v1.16b
      | ^
4 errors generated.
[10940/63891] aarch64-unknown-linux-gnu-clang++-18 -MD -MF obj/third_party/libyuv/libyuv_neon/rotate_neon.o.d

Seems because some -march/-mcpu/-mtune drop ‘dotprod’ parameters, it needs to add the extension manuall. ‘+dotprod’.

Edit the file /etc/portage/make.conf, and change the COMMON_FLAGS to

COMMON_FLAGS="-mcpu=neoverse-n1+crc+crypto+ssbs+dotprod -O2 -pipe"

It can use the following command to get current CPU flag

gcc -v -E -x c /dev/null -o /dev/null -march=native -mcpu=native 2>&1 | grep /cc1

Thanks Gentoo user zukunf to provide those information.