# \[RFC\] Add "hot" function attribute to LLVM IR and use hot/cold attribute in function section prefix

**URL:** <https://discourse.llvm.org/t/rfc-add-hot-function-attribute-to-llvm-ir-and-use-hot-cold-attribute-in-function-section-prefix/57103>\
**Category:** LLVM Dev List Archives\
**Created:** [December 4, 2020, 1:41am UTC](https://discourse.llvm.org/t/rfc-add-hot-function-attribute-to-llvm-ir-and-use-hot-cold-attribute-in-function-section-prefix/57103 "2020-12-04T01:41:15Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![xur](https://sea1.discourse-cdn.com/flex021/user_avatar/discourse.llvm.org/xur/32/22173_2.png) [@xur](https://discourse.llvm.org/u/xur)\
**Post date:** [December 4, 2020, 1:41am UTC](https://discourse.llvm.org/t/rfc-add-hot-function-attribute-to-llvm-ir-and-use-hot-cold-attribute-in-function-section-prefix/57103/1 "2020-12-04T01:41:15Z")

</div>

[RFC] Add “hot” function attribute to LLVM IR and use hot/cold attribute  
in function section prefix

**attribute** ((hot)) and **attribute** ((cold)) are supported in Clang.  
Only the “cold” attribute is passed down to LLVM IR. “hot” attribute is  
silently dropped by Clang, as currently only have cold function attribute  
in LLVM IR.

These two attributes are not documented in clang reference manual:  
[https://clang.llvm.org/docs/AttributeReference.html](https://clang.llvm.org/docs/AttributeReference.html)  
They are here mainly for gcc-compatibility:  
[https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html](https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html)

We propose to add a “hot” function attribute to LLVM IR. Like the “cold”  
function attribute, “hot” attribute can be used in optimization passes.

The immediate usage we are having is to set the function section prefix/suffix.  
Currently “.hot” and “.unlikely” suffix only are added in PGO (or Sample PGO)  
compilation (through isFunctionHotInCallGraph and isFunctionColdInCallGraph).

With the hot/cold function attribute, the new behavior is:  
(1) If the user annotates a function as hot or isFunctionHotInCallGraph  
is true, this function will be marked as hot.  
(2) If the user annotates a function as cold or isFunctionColdInCallGraph  
is true, this function will be marked as cold.

The changes are:  
(1) user annotation will be used in setting function section prefix/suffix.  
(2) hot attribute overwrites profile count based hotness.  
(3) profile count based hotness overwrite user annotated cold attribute.

The intention for these changes is to provide the user a way to mark  
certain functions as hot in cases where training input is hard to cover all  
the hot functions.

Note that with this change, the attributes will have a different semantic  
from GCC, where PGO profile hotness always has precedence over the function  
attribute annotation.

Currently the cold function attribute is also set in PGO use pass. The code is before isFunctionColdInCallGraph and does not use PSI. We might want to change that code  
to make this attribute as user-annotation only.

Comments and feedback are welcome. The patch for this RFC is:  
[https://reviews.llvm.org/D92493](https://reviews.llvm.org/D92493)

Thanks,

-Rong