================
@@ -16303,6 +16303,56 @@ Example:
call void @llvm.call.preallocated.teardown(token %cs)
ret void
+.. _int_asm_constraint_br:
+
+'``llvm.asm.constraint.br``'
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+ declare void @llvm.asm.constraint.br()
+
+Overview:
+"""""""""
+
+The '``llvm.asm.constraint.br``' intrinsic is used when an inline asm
+constraint allows for either register or memory, e.g., '``"rm"``'.
+
+Semantics:
+""""""""""
+
+The '``llvm.asm.constraint.br``' allows the back-end to choose the best
+constraint rather than restricting the preferred constraint to one that may
+produce substandard code or cannot be handled by the register allocators.
+
+It can be called only by the '``callbr``' instruction. The default destination
+of the ``callbr`` contains a call to the preferred inline asm, while the single
+indirect destination contains a call to the pessimal inline asm.
+
+Example:
+""""""""
+
+.. code-block:: llvm
+
+ %out = alloca i64, align 8
+ callbr void @llvm.asm.constraint.br()
+ to label %asm.pref.reg [label %asm.pref.mem]
+
+ asm.pref.reg:
+ %0 = tail call i64 asm sideeffect "pushf ; pop $0",
"=rm,~{dirflag},~{fpsr},~{flags}"()
+ br label %asm.merge
+
+ asm.pref.mem:
+ call void asm sideeffect "pushf ; pop $0",
"=*rm,~{dirflag},~{fpsr},~{flags}"(ptr nonnull elementtype(i64) %out)
----------------
nickdesaulniers wrote:
I feel like it might be worth noting in this case that we _could_ remove the
`r` from the constraint in this Instruction, but then we wouldn't have a simple
reuse of codegen from the front end. Not sure where that comment best belongs,
perhaps where we do the clang/lib/CodeGen/* ?
https://github.com/llvm/llvm-project/pull/197798
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