[llvm] IR: Add prefalign attribute for function definitions. (PR #155527)

[Eli Friedman via llvm-commits]

================
@@ -942,11 +944,24 @@ LLVM allows an explicit section to be specified for functions. If the
 target supports it, it will emit functions to the section specified.
 Additionally, the function can be placed in a COMDAT.
 
-An explicit alignment may be specified for a function. If not present,
-or if the alignment is set to zero, the alignment of the function is set
-by the target to whatever it feels convenient. If an explicit alignment
-is specified, the function is forced to have at least that much
-alignment. All alignments must be a power of 2.
+An explicit minimum alignment (``align``) may be specified for a
+function. If not present, or if the alignment is set to zero, the
+alignment of the function is set according to the preferred alignment
+rules described below. If an explicit minimum alignment is specified, the
+function is forced to have at least that much alignment. All alignments
+must be a power of 2.
+
+An explicit preferred alignment (``prefalign``) may also be specified
+for a function (definitions only, and must be a power of 2). If a
+function does not have a preferred alignment attribute, the preferred
+alignment is determined in a target-specific way. The final alignment
+of the function is determined in the following way: if the function
+size is less than the minimum alignment, the function's alignment will
+be at least the minimum alignment. Otherwise, if the function size is
+between the minimum alignment and the preferred alignment, the function's
+alignment will be at least the power of 2 greater than or equal to the
+function size. Otherwise, the function's alignment will be at least the
+preferred alignment.
----------------
efriedma-quic wrote:

This seems overly specific: if you read it literally, it means that we have to precisely compute the size of the function, which is probably not feasible in general, and that optimizations aren't allowed to modify prefalign.  I think I'd prefer to say it's a hint, which can be overriden by PGO/CFI jump tables/etc.

https://github.com/llvm/llvm-project/pull/155527