[Qemu-devel] [PATCH RFC 9/9] tcg: update README about size changing ops

[Aurelien Jarno <aurelien@aurel32.net>]

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 tcg/README | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tcg/README b/tcg/README
index 61b3899..d8fd17a 100644
--- a/tcg/README
+++ b/tcg/README
@@ -470,8 +470,19 @@ Floating point operations are not supported in this version. A
 previous incarnation of the code generator had full support of them,
 but it is better to concentrate on integer operations first.
 
-On a 64 bit target, no assumption is made in TCG about the storage of
-the 32 bit values in 64 bit registers.
+On a 64 bit target, the values are transfered between 32 and 64-bit
+registers by the mean of the following ops:
+- trunc_shr_i64_i32
+- ext_i32_i64
+- extu_i32_i64
+
+These ops are all optional in that case they are implemented as mov.
+This is to allow some optimizations if the target maintains registers
+zero or sign extended. For example a MIPS64 CPU requires that all
+32-bit values are stored sign-extended in the registers. This means
+the trunc_shr_i64_i32 should sign-extend the value when moving it
+from a 64-bit to a 32-bit register. It also means ext_i32_i64 can be
+implemented as a simple mov as the value is already sign extended.
 
 4.2) Constraints
 
-- 
2.1.4