A TCG "basic block" corresponds to a list of instructions terminated
by a branch instruction.
+An operation with "undefined behavior" may result in a crash.
+
+An operation with "unspecified behavior" shall not crash. However,
+the result may be one of several possibilities so may be considered
+an "undefined result".
+
3) Intermediate representation
3.1) Introduction
* shl_i32/i64 t0, t1, t2
-t0=t1 << t2. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 << t2. Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* shr_i32/i64 t0, t1, t2
-t0=t1 >> t2 (unsigned). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 >> t2 (unsigned). Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* sar_i32/i64 t0, t1, t2
-t0=t1 >> t2 (signed). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+t0=t1 >> t2 (signed). Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* rotl_i32/i64 t0, t1, t2
-Rotation of t2 bits to the left. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+Rotation of t2 bits to the left.
+Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
* rotr_i32/i64 t0, t1, t2
-Rotation of t2 bits to the right. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)
+Rotation of t2 bits to the right.
+Unspecified behavior if t2 < 0 or t2 >= 32 (resp 64)
********* Misc
dest = (t1 & ~0x0f00) | ((t2 << 8) & 0x0f00)
+* extrl_i64_i32 t0, t1
+
+For 64-bit hosts only, extract the low 32-bits of input T1 and place it
+into 32-bit output T0. Depending on the host, this may be a simple move,
+or may require additional canonicalization.
+
+* extrh_i64_i32 t0, t1
+
+For 64-bit hosts only, extract the high 32-bits of input T1 and place it
+into 32-bit output T0. Depending on the host, this may be a simple shift,
+or may require additional canonicalization.
********* Conditional moves
instructions. Only indices 0 and 1 are valid and tcg_gen_goto_tb may be issued
at most once with each slot index per TB.
-* qemu_ld8u t0, t1, flags
-qemu_ld8s t0, t1, flags
-qemu_ld16u t0, t1, flags
-qemu_ld16s t0, t1, flags
-qemu_ld32 t0, t1, flags
-qemu_ld32u t0, t1, flags
-qemu_ld32s t0, t1, flags
-qemu_ld64 t0, t1, flags
+* qemu_ld_i32/i64 t0, t1, flags, memidx
+* qemu_st_i32/i64 t0, t1, flags, memidx
+
+Load data at the guest address t1 into t0, or store data in t0 at guest
+address t1. The _i32/_i64 size applies to the size of the input/output
+register t0 only. The address t1 is always sized according to the guest,
+and the width of the memory operation is controlled by flags.
-Load data at the QEMU CPU address t1 into t0. t1 has the QEMU CPU address
-type. 'flags' contains the QEMU memory index (selects user or kernel access)
-for example.
+Both t0 and t1 may be split into little-endian ordered pairs of registers
+if dealing with 64-bit quantities on a 32-bit host.
-Note that "qemu_ld32" implies a 32-bit result, while "qemu_ld32u" and
-"qemu_ld32s" imply a 64-bit result appropriately extended from 32 bits.
+The memidx selects the qemu tlb index to use (e.g. user or kernel access).
+The flags are the TCGMemOp bits, selecting the sign, width, and endianness
+of the memory access.
-* qemu_st8 t0, t1, flags
-qemu_st16 t0, t1, flags
-qemu_st32 t0, t1, flags
-qemu_st64 t0, t1, flags
+For a 32-bit host, qemu_ld/st_i64 is guaranteed to only be used with a
+64-bit memory access specified in flags.
-Store the data t0 at the QEMU CPU Address t1. t1 has the QEMU CPU
-address type. 'flags' contains the QEMU memory index (selects user or
-kernel access) for example.
+*********
Note 1: Some shortcuts are defined when the last operand is known to be
a constant (e.g. addi for add, movi for mov).
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
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