tcg: update README about size changing ops

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>

tcg/README

@@ -466,13 +466,25 @@ On a 32 bit target, all 64 bit operations are converted to 32 bits. A
 few specific operations must be implemented to allow it (see add2_i32,
 sub2_i32, brcond2_i32).
 
+On a 64 bit target, the values are transfered between 32 and 64-bit
+registers using the following ops:
+- trunc_shr_i64_i32
+- ext_i32_i64
+- extu_i32_i64
+
+They ensure that the values are correctly truncated or extended when
+moved from a 32-bit to a 64-bit register or vice-versa. Note that the
+trunc_shr_i64_i32 is an optional op. It is not necessary to implement
+it if all the following conditions are met:
+- 64-bit registers can hold 32-bit values
+- 32-bit values in a 64-bit register do not need to stay zero or
+  sign extended
+- all 32-bit TCG ops ignore the high part of 64-bit registers
+
 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.
-
 4.2) Constraints
 
 GCC like constraints are used to define the constraints of every