Add 'cold' attribute to functions

[Diego Novillo]

On 2013-04-11 09:18 , Duncan Sands wrote:
> Hi Diego,
>
> On 10/04/13 23:33, Diego Novillo wrote:
>> This patch adds a new function attribute 'cold' to functions.
>
> can you please add a testcase or two checking that this works properly
> when the callsite has the 'cold' attribute but the called function does
> not.  (It should work automagically, but it would be best to be sure).
> This could be useful if the function is not always cold, but some calls
> to it are known cold by the front-end somehow.

Sure.  Interestingly, the analysis triggers correctly on the following:

[ ... ]
entry:
   %gep1 = getelementptr i32* %a, i32 1
   %val1 = load i32* %gep1
   %cond1 = icmp ugt i32 %val1, 1
   br i1 %cond1, label %then, label %else

then:
   ; This function is not declared cold, but this call site is.
   %val4 = call i32 @regular_function(i32 %val1) cold
   br label %exit

else:
   %gep2 = getelementptr i32* %a, i32 2
   %val2 = load i32* %gep2
   %val3 = call i32 @regular_function(i32 %val2)
   br label %exit
[ ... ]

The entry->else is marked hot with a high weight.  However, code 
generation still merges the calls to 'regular_function' in the two 
branches, which makes code layout put the call at the bottom.  I think 
this should not be happening, though.

         .globl  test_cold_call_sites
         .align  16, 0x90
         .type   test_cold_call_sites, at function
test_cold_call_sites:                   # @test_cold_call_sites
         .cfi_startproc
# BB#0:                                 # %entry
         pushq   %rax.Ltmp71:
         .cfi_def_cfa_offset 16
         movq    %rdi, %rax
         movl    4(%rax), %edi
         cmpl    $2, %edi
         jae     .LBB22_2
# BB#1:                                 # %else
         movl    8(%rax), %edi
.LBB22_2:                               # %else
         callq   regular_function
         popq    %rdx
         ret


Where should I look into this?


I've added an analysis test, but I have not added a code gen test for 
it.  I need to figure out whether this can be changed.


Thanks.  Diego.