Using the tcg_gen_helper_x_y it is possible to call any function
taking i32, i64 or pointer types. By default, before calling a helper,
all globals are stored at their canonical location and it is assumed
-that the function can modify them. This can be overridden by the
-TCG_CALL_CONST function modifier. By default, the helper is allowed to
-modify the CPU state or raise an exception. This can be overridden by
-the TCG_CALL_PURE function modifier, in which case the call to the
-function is removed if the return value is not used.
+that the function can modify them. By default, the helper is allowed to
+modify the CPU state or raise an exception.
+
+This can be overridden using the following function modifiers:
+- TCG_CALL_NO_READ_GLOBALS means that the helper does not read globals,
+ either directly or via an exception. They will not be saved to their
+ canonical locations before calling the helper.
+- TCG_CALL_NO_WRITE_GLOBALS means that the helper does not modify any globals.
+ They will only be saved to their canonical location before calling helpers,
+ but they won't be reloaded afterwise.
+- TCG_CALL_NO_SIDE_EFFECTS means that the call to the function is removed if
+ the return value is not used.
+
+Note that TCG_CALL_NO_READ_GLOBALS implies TCG_CALL_NO_WRITE_GLOBALS.
On some TCG targets (e.g. x86), several calling conventions are
supported.
* Branches:
-Use the instruction 'br' to jump to a label. Use 'jmp' to jump to an
-explicit address. Conditional branches can only jump to labels.
+Use the instruction 'br' to jump to a label.
3.3) Code Optimizations
********* Jumps/Labels
-* jmp t0
-
-Absolute jump to address t0 (pointer type).
-
* set_label $label
Define label 'label' at the current program point.
Jump to label.
-* brcond_i32/i64 cond, t0, t1, label
+* brcond_i32/i64 t0, t1, cond, label
Conditional jump if t0 cond t1 is true. cond can be:
TCG_COND_EQ
Indicate that the value of t0 won't be used later. It is useful to
force dead code elimination.
-* deposit_i32/i64 dest, t1, t2, pos, loc
+* deposit_i32/i64 dest, t1, t2, pos, len
Deposit T2 as a bitfield into T1, placing the result in DEST.
-The bitfield is described by POS/LOC, which are immediate values:
+The bitfield is described by POS/LEN, which are immediate values:
LEN - the length of the bitfield
POS - the position of the first bit, counting from the LSB
********* Conditional moves
-* setcond_i32/i64 cond, dest, t1, t2
+* setcond_i32/i64 dest, t1, t2, cond
dest = (t1 cond t2)
Set DEST to 1 if (T1 cond T2) is true, otherwise set to 0.
+* movcond_i32/i64 dest, c1, c2, v1, v2, cond
+
+dest = (c1 cond c2 ? v1 : v2)
+
+Set DEST to V1 if (C1 cond C2) is true, otherwise set to V2.
+
********* Type conversions
* ext_i32_i64 t0, t1
write(t0, t1 + offset)
Write 8, 16, 32 or 64 bits to host memory.
+All this opcodes assume that the pointed host memory doesn't correspond
+to a global. In the latter case the behaviour is unpredictable.
+
********* 64-bit target on 32-bit host support
The following opcodes are internal to TCG. Thus they are to be implemented by
32-bit host code generators, but are not to be emitted by guest translators.
They are emitted as needed by inline functions within "tcg-op.h".
-* brcond2_i32 cond, t0_low, t0_high, t1_low, t1_high, label
+* brcond2_i32 t0_low, t0_high, t1_low, t1_high, cond, label
Similar to brcond, except that the 64-bit values T0 and T1
are formed from two 32-bit arguments.
Similar to mul, except two 32-bit (unsigned) inputs T1 and T2 yielding
the full 64-bit product T0. The later is returned in two 32-bit outputs.
-* setcond2_i32 cond, dest, t1_low, t1_high, t2_low, t2_high
+* setcond2_i32 dest, t1_low, t1_high, t2_low, t2_high, cond
Similar to setcond, except that the 64-bit values T1 and T2 are
formed from two 32-bit arguments. The result is a 32-bit value.
Exit the current TB and jump to the TB index 'index' (constant) if the
current TB was linked to this TB. Otherwise execute the next
-instructions.
+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
- Don't hesitate to use helpers for complicated or seldom used target
instructions. There is little performance advantage in using TCG to
implement target instructions taking more than about twenty TCG
- instructions.
+ instructions. Note that this rule of thumb is more applicable to
+ helpers doing complex logic or arithmetic, where the C compiler has
+ scope to do a good job of optimisation; it is less relevant where
+ the instruction is mostly doing loads and stores, and in those cases
+ inline TCG may still be faster for longer sequences.
+
+- The hard limit on the number of TCG instructions you can generate
+ per target instruction is set by MAX_OP_PER_INSTR in exec-all.h --
+ you cannot exceed this without risking a buffer overrun.
- Use the 'discard' instruction if you know that TCG won't be able to
prove that a given global is "dead" at a given program point. The
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