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some asm block flags also mean there can be no fences #1413

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merged 2 commits into from
Nov 4, 2023
Merged

some asm block flags also mean there can be no fences #1413

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fae136c
nomem, readonly also means no fences
RalfJung Oct 9, 2023
c7690f1
pure requires readonly or nomem
RalfJung Oct 17, 2023
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4 changes: 3 additions & 1 deletion src/inline-assembly.md
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Expand Up @@ -414,10 +414,13 @@ Flags are used to further influence the behavior of the inline assembly block.
Currently the following options are defined:
- `pure`: The `asm!` block has no side effects, and its outputs depend only on its direct inputs (i.e. the values themselves, not what they point to) or values read from memory (unless the `nomem` options is also set).
This allows the compiler to execute the `asm!` block fewer times than specified in the program (e.g. by hoisting it out of a loop) or even eliminate it entirely if the outputs are not used.
The `pure` option must be combined with either the `nomem` or `readonly` options, otherwise a compile-time error is emitted.
- `nomem`: The `asm!` blocks does not read or write to any memory.
This allows the compiler to cache the values of modified global variables in registers across the `asm!` block since it knows that they are not read or written to by the `asm!`.
The compiler also assumes that this `asm!` block does not perform any kind of synchronization with other threads, e.g. via fences.
- `readonly`: The `asm!` block does not write to any memory.
This allows the compiler to cache the values of unmodified global variables in registers across the `asm!` block since it knows that they are not written to by the `asm!`.
The compiler also assumes that this `asm!` block does not perform any kind of synchronization with other threads, e.g. via fences.
- `preserves_flags`: The `asm!` block does not modify the flags register (defined in the rules below).
This allows the compiler to avoid recomputing the condition flags after the `asm!` block.
- `noreturn`: The `asm!` block never returns, and its return type is defined as `!` (never).
Expand All @@ -432,7 +435,6 @@ Currently the following options are defined:

The compiler performs some additional checks on options:
- The `nomem` and `readonly` options are mutually exclusive: it is a compile-time error to specify both.
- The `pure` option must be combined with either the `nomem` or `readonly` options, otherwise a compile-time error is emitted.
- It is a compile-time error to specify `pure` on an asm block with no outputs or only discarded outputs (`_`).
- It is a compile-time error to specify `noreturn` on an asm block with outputs.

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