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[LangRef] asm clobber constrains: '~memory' allows reads and synchronization #150191

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[LangRef] asm clobber constrains: '~memory' allows reads and synchronization #150191

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11 changes: 7 additions & 4 deletions llvm/docs/LangRef.rst
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Expand Up @@ -5448,10 +5448,13 @@ Clobber constraints
A clobber constraint is indicated by a "``~``" prefix. A clobber does not
consume an input operand, nor generate an output. Clobbers cannot use any of the
general constraint code letters -- they may use only explicit register
constraints, e.g. "``~{eax}``". The one exception is that a clobber string of
"``~{memory}``" indicates that the assembly writes to arbitrary undeclared
memory locations -- not only the memory pointed to by a declared indirect
output.
constraints, e.g. "``~{eax}``".

The one exception is that a clobber string of "``~{memory}``" indicates that the
assembly reads and writes arbitrary undeclared memory locations -- not only the
memory pointed to by a declared indirect output. Furthermore, the assembly may
also cause synchronization with other threads, such as via release/acquire
fences and atomic memory accesses.

Note that clobbering named registers that are also present in output
constraints is not legal.
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