doc: RISC-V: Document that -mcpu doesn't override -march or -mtune

This came up recently as Edwin was looking through the test suite.  A
few of us were talking about this during the patchwork meeting and were
surprised.  Looks like this is the desired behavior, so let's at least
document it.

gcc/ChangeLog:

	* doc/invoke.texi: Document -mcpu.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 58527e1ea3c5cb5ed778858d00ac54fdea071ef5..4f4ca2b76720d6c05c364f899179b6fa842e1be5 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -30750,6 +30750,8 @@ Permissible values for this option are: @samp{sifive-e20}, @samp{sifive-e21},
 @samp{sifive-u54}, @samp{sifive-u74}, @samp{sifive-x280}, @samp{sifive-xp450},
 @samp{sifive-x670}.
 
+Note that @option{-mcpu} does not override @option{-march} or @option{-mtune}.
+
 @opindex mtune
 @item -mtune=@var{processor-string}
 Optimize the output for the given processor, specified by microarchitecture or