x86: document certain command line options as "dangerous"

Errata BT36, BT41, and BT230 mean that gas may, when using one ofthese
options, produce code that causes #UD on (at least) SandyBridge systems.

1 files changed, 6 insertions, 0 deletions

diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi
index 9d821ae..908c6ca 100644
--- a/gas/doc/c-i386.texi
+++ b/gas/doc/c-i386.texi
@@ -303,6 +303,9 @@ AVX instructions with 128bit vector length, which is the default.
 @option{-mavxscalar=@var{256}} will encode scalar AVX instructions
 with 256bit vector length.
 
+WARNING: Don't use this for production code - due to CPU errata the
+resulting code may not work on certain models.
+
 @cindex @samp{-mvexwig=} option, i386
 @cindex @samp{-mvexwig=} option, x86-64
 @item -mvexwig=@var{0}
@@ -313,6 +316,9 @@ instructions with vex.w = 0, which is the default.
 @option{-mvexwig=@var{1}} will encode WIG EVEX instructions with
 vex.w = 1.
 
+WARNING: Don't use this for production code - due to CPU errata the
+resulting code may not work on certain models.
+
 @cindex @samp{-mevexlig=} option, i386
 @cindex @samp{-mevexlig=} option, x86-64
 @item -mevexlig=@var{128}