[llvm-bugs] [Bug 28441] New: common predicate value is not hoisted

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at 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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