[PATCH] invoke.texi: Clarify -march=lujiazui

[Jakub Jelinek <jakub@redhat.com>]

Hi!

Yesterday I was searching which exact CPUs are affected by the PR114576
wrong-code issue and went from the PTA_* bitmasks in GCC, so arrived
at the goldmont, goldmont-plus, tremont and lujiazui CPUs (as -march=
cases which do enable -maes and don't enable -mavx).
But when double-checking that against the invoke.texi documentation,
that was true for the first 3, but lujiazui said it supported AVX.
I was really confused by that, until I found the
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604407.html
explanation.  So, seems the CPUs do have AVX and F16C but -march=lujiazui
doesn't enable those and even activelly attempts to filter those out from
the announced CPUID features, in glibc as well as e.g. in libgcc.

Thus, I think we should document what actually happens, otherwise
users could assume that
gcc -march=lujiazui predefines __AVX__ and __F16C__, which it doesn't.

Tested on x86_64, ok for trunk?

2024-04-11  Jakub Jelinek  <jakub@redhat.com>

	* doc/invoke.texi (lujiazui): Clarify that while the CPUs do support
	AVX and F16C, -march=lujiazui actually doesn't enable those.

--- gcc/doc/invoke.texi.jj	2024-04-11 09:26:01.156865894 +0200
+++ gcc/doc/invoke.texi	2024-04-11 10:47:53.457582922 +0200
@@ -34696,8 +34696,10 @@ instruction set support.
 
 @item lujiazui
 ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
-SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16,
-ABM, BMI, BMI2, F16C, FXSR, RDSEED instruction set support.
+SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16,
+ABM, BMI, BMI2, FXSR, RDSEED instruction set support.  While the CPUs
+do support AVX and F16C, these aren't enabled by @code{-march=lujiazui}
+for performance reasons.
 
 @item yongfeng
 ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,

	Jakub