tcg: Allow an operand to be matching or a constant

This allows an output operand to match an input operand
only when the input operand needs a register.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

1 files changed, 9 insertions, 4 deletions

diff --git a/tcg/README b/tcg/README
index 065d9c2966..6946b5bcc2 100644
--- a/tcg/README
+++ b/tcg/README
@@ -539,24 +539,29 @@ version. Aliases are specified in the input operands as for GCC.
 The same register may be used for both an input and an output, even when
 they are not explicitly aliased.  If an op expands to multiple target
 instructions then care must be taken to avoid clobbering input values.
-GCC style "early clobber" outputs are not currently supported.
+GCC style "early clobber" outputs are supported, with '&'.
 
 A target can define specific register or constant constraints. If an
 operation uses a constant input constraint which does not allow all
 constants, it must also accept registers in order to have a fallback.
+The constraint 'i' is defined generically to accept any constant.
+The constraint 'r' is not defined generically, but is consistently
+used by each backend to indicate all registers.
 
 The movi_i32 and movi_i64 operations must accept any constants.
 
 The mov_i32 and mov_i64 operations must accept any registers of the
 same type.
 
-The ld/st instructions must accept signed 32 bit constant offsets. It
-can be implemented by reserving a specific register to compute the
-address if the offset is too big.
+The ld/st/sti instructions must accept signed 32 bit constant offsets.
+This can be implemented by reserving a specific register in which to
+compute the address if the offset is too big.
 
 The ld/st instructions must accept any destination (ld) or source (st)
 register.
 
+The sti instruction may fail if it cannot store the given constant.
+
 4.3) Function call assumptions
 
 - The only supported types for parameters and return value are: 32 and