https://llvm.org/bugs/show_bug.cgi?id=26636
Bug ID: 26636
Summary: [SimplifyCFG] branch weights are not preserved when
forming select
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Transformation Utilities
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
The conditional branch has profile weight metadata:
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"
define i32 @foo(i32 %x) #0 {
entry:
%cmp = icmp sgt i32 %x, 4
br i1 %cmp, label %if.then, label %if.end, !prof !0
if.then:
%add = add nsw i32 %x, 7
br label %return
if.end:
%mul = mul nsw i32 %x, 17
br label %return
return:
%retval.0 = phi i32 [ %add, %if.then ], [ %mul, %if.end ]
ret i32 %retval.0
}
!0 = !{!"branch_weights", i32 676, i32 207}
-------------------------------------------------------------------------------
But after -simplifycfg, the metadata is gone:
$ ./opt -simplifycfg selbrweight.ll -S
; ModuleID = 'selbrweight.ll'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"
define i32 @foo(i32 %x) {
entry:
%cmp = icmp sgt i32 %x, 4
%add = add nsw i32 %x, 7
%mul = mul nsw i32 %x, 17
%retval.0 = select i1 %cmp, i32 %add, i32 %mul
ret i32 %retval.0
}
-------------------------------------------------------------------------------
I don't know if weight metadata can currently be applied to selects, but the
select instruction should propagate the weight info that was on the branch.
This was noted by Benjamin Kramer in:
http://reviews.llvm.org/D16836
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