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| -rw-r--r-- | tcg/README | 14 |
1 files changed, 9 insertions, 5 deletions
diff --git a/tcg/README b/tcg/README index 934e7afc96..063aeb95ea 100644 --- a/tcg/README +++ b/tcg/README @@ -14,6 +14,10 @@ the emulated architecture. As TCG started as a generic C backend used for cross compiling, it is assumed that the TCG target is different from the host, although it is never the case for QEMU. +In this document, we use "guest" to specify what architecture we are +emulating; "target" always means the TCG target, the machine on which +we are running QEMU. + A TCG "function" corresponds to a QEMU Translated Block (TB). A TCG "temporary" is a variable only live in a basic @@ -379,7 +383,7 @@ double-word product T0. The later is returned in two single-word outputs. Similar to mulu2, except the two inputs T1 and T2 are signed. -********* 64-bit target on 32-bit host support +********* 64-bit guest 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. @@ -521,9 +525,9 @@ register. a better generated code, but it reduces the memory usage of TCG and the speed of the translation. -- Don't hesitate to use helpers for complicated or seldom used target +- Don't hesitate to use helpers for complicated or seldom used guest instructions. There is little performance advantage in using TCG to - implement target instructions taking more than about twenty TCG + implement guest instructions taking more than about twenty TCG 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 @@ -531,9 +535,9 @@ register. 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 -- + per guest 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 - x86 target uses it to improve the condition codes optimisation. + x86 guest uses it to improve the condition codes optimisation. |