https://llvm.org/bugs/show_bug.cgi?id=28441
Bug ID: 28441
Summary: common predicate value is not hoisted
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
extern void bar(int);
void f(int x, int y) {
if (x && y == 1)
bar(8000);
if (x && y == 2)
bar(9000);
}
ICC checks 'x' first and effectively turns this code into:
void g(int x, int y) {
if (x) {
if (y == 1)
bar(8000);
if (y == 2)
bar(9000);
}
}
LLVM does not, and the code size penalty is substantial (not sure if perf is
better, but seems like it shouldn't be worse):
$ clang ifand.c -O1 -fomit-frame-pointer -c
$ otool -tv ifand.o
_f:
0000000000000000 pushq %rbp
0000000000000001 pushq %rbx
0000000000000002 pushq %rax
0000000000000003 movl %esi, %ebx
0000000000000005 movl %edi, %ebp
0000000000000007 testl %ebp, %ebp
0000000000000009 je 0x1a
000000000000000b cmpl $0x1, %ebx
000000000000000e jne 0x1a
0000000000000010 movl $0x1f40, %edi ## imm = 0x1F40
0000000000000015 callq 0x1a
000000000000001a testl %ebp, %ebp
000000000000001c je 0x33
000000000000001e cmpl $0x2, %ebx
0000000000000021 jne 0x33
0000000000000023 movl $0x2328, %edi ## imm = 0x2328
0000000000000028 addq $0x8, %rsp
000000000000002c popq %rbx
000000000000002d popq %rbp
000000000000002e jmp 0x33
0000000000000033 addq $0x8, %rsp
0000000000000037 popq %rbx
0000000000000038 popq %rbp
0000000000000039 retq
000000000000003a nopw (%rax,%rax)
_g:
0000000000000040 testl %edi, %edi
0000000000000042 je 0x58
0000000000000044 cmpl $0x2, %esi
0000000000000047 je 0x59
0000000000000049 cmpl $0x1, %esi
000000000000004c jne 0x58
000000000000004e movl $0x1f40, %edi ## imm = 0x1F40
0000000000000053 jmp 0x58
0000000000000058 retq
0000000000000059 movl $0x2328, %edi ## imm = 0x2328
000000000000005e jmp 0x63
Even if there's disagreement on which form is better, I think the IR should be
canonicalized to one or the other, allowing the backend to optimize whichever
way it wants to:
$ ./clang ifand.c -O1 -fomit-frame-pointer -S -o - -emit-llvm
; ModuleID = 'ifand.c'
source_filename = "ifand.c"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"
; Function Attrs: nounwind ssp uwtable
define void @f(i32 %x, i32 %y) local_unnamed_addr #0 {
entry:
%tobool = icmp ne i32 %x, 0
%cmp = icmp eq i32 %y, 1
%or.cond = and i1 %tobool, %cmp
br i1 %or.cond, label %if.then, label %if.end
if.then: ; preds = %entry
tail call void @bar(i32 8000) #2
br label %if.end
if.end: ; preds = %if.then, %entry
%cmp3 = icmp eq i32 %y, 2
%or.cond6 = and i1 %tobool, %cmp3
br i1 %or.cond6, label %if.then4, label %if.end5
if.then4: ; preds = %if.end
tail call void @bar(i32 9000) #2
br label %if.end5
if.end5: ; preds = %if.then4, %if.end
ret void
}
declare void @bar(i32) local_unnamed_addr #1
; Function Attrs: nounwind ssp uwtable
define void @g(i32 %x, i32 %y) local_unnamed_addr #0 {
entry:
%tobool = icmp eq i32 %x, 0
br i1 %tobool, label %if.end5, label %if.then
if.then: ; preds = %entry
switch i32 %y, label %if.end5 [
i32 1, label %if.then1
i32 2, label %if.then3
]
if.then1: ; preds = %if.then
tail call void @bar(i32 8000) #2
br label %if.end5
if.then3: ; preds = %if.then
tail call void @bar(i32 9000) #2
br label %if.end5
if.end5: ; preds = %if.then1,
%if.then, %entry, %if.then3
ret void
}
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