[llvm-dev] Code optimisation regression(?) in loops compiled with the -Oz flag

[Joan Lluch via llvm-dev]

Hi All,

This simple loop code

void loopTest()
{
  for ( int i = 0 ; i<10 ; i++ ) {
    ftest();
  }
}

gets converted into this when compiled with the -Os flags:

; Function Attrs: nounwind optsize uwtable
define void @loopTest() local_unnamed_addr #0 {
entry:
  br label %for.body

for.cond.cleanup:                                 ; preds = %for.body
  ret void

for.body:                                         ; preds = %for.body, %entry
  %i.03 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
  %call = tail call i32 (...) @ftest() #2
  %inc = add nuw nsw i32 %i.03, 1
  %exitcond = icmp eq i32 %inc, 10
  br i1 %exitcond, label %for.cond.cleanup, label %for.body
}

For the x86 architecture, this gets compiled into this

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 12
	.globl	_loopTest               ## -- Begin function loopTest
_loopTest:                              ## @loopTest
	.cfi_startproc
## %bb.0:                               ## %entry
	pushq	%rbp                    ## encoding: [0x55]
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp              ## encoding: [0x48,0x89,0xe5]
	.cfi_def_cfa_register %rbp
	pushq	%rbx                    ## encoding: [0x53]
	pushq	%rax                    ## encoding: [0x50]
	.cfi_offset %rbx, -24
	movl	$10, %ebx               ## encoding: [0xbb,0x0a,0x00,0x00,0x00]
LBB0_1:                                 ## %for.body
                                        ## =>This Inner Loop Header: Depth=1
	xorl	%eax, %eax              ## encoding: [0x31,0xc0]
	callq	_ftest                  ## encoding: [0xe8,A,A,A,A]
                                        ##   fixup A - offset: 1, value: _ftest-4, kind: reloc_branch_4byte_pcrel
	decl	%ebx                    ## encoding: [0xff,0xcb]
	jne	LBB0_1                  ## encoding: [0x75,A]
                                        ##   fixup A - offset: 1, value: LBB0_1-1, kind: FK_PCRel_1
## %bb.2:                               ## %for.cond.cleanup
	addq	$8, %rsp                ## encoding: [0x48,0x83,0xc4,0x08]
	popq	%rbx                    ## encoding: [0x5b]
	popq	%rbp                    ## encoding: [0x5d]
	retq                            ## encoding: [0xc3]
	.cfi_endproc
                                        ## -- End function



The same code compiled with -Oz results in the following:

; Function Attrs: minsize nounwind optsize uwtable
define void @loopTest() local_unnamed_addr #0 {
entry:
  br label %for.cond

for.cond:                                         ; preds = %for.body, %entry
  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
  %exitcond = icmp eq i32 %i.0, 10
  br i1 %exitcond, label %for.cond.cleanup, label %for.body

for.cond.cleanup:                                 ; preds = %for.cond
  ret void

for.body:                                         ; preds = %for.cond
  %call = tail call i32 (...) @ftest() #2
  %inc = add nuw nsw i32 %i.0, 1
  br label %for.cond
}

and this

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 12
	.globl	_loopTest               ## -- Begin function loopTest
_loopTest:                              ## @loopTest
	.cfi_startproc
## %bb.0:                               ## %entry
	pushq	%rbp                    ## encoding: [0x55]
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp              ## encoding: [0x48,0x89,0xe5]
	.cfi_def_cfa_register %rbp
	pushq	%rbx                    ## encoding: [0x53]
	pushq	%rax                    ## encoding: [0x50]
	.cfi_offset %rbx, -24
	pushq	$10                     ## encoding: [0x6a,0x0a]
	popq	%rbx                    ## encoding: [0x5b]
LBB0_1:                                 ## %for.cond
                                        ## =>This Inner Loop Header: Depth=1
	testl	%ebx, %ebx              ## encoding: [0x85,0xdb]
	je	LBB0_2                  ## encoding: [0x74,A]
                                        ##   fixup A - offset: 1, value: LBB0_2-1, kind: FK_PCRel_1
## %bb.3:                               ## %for.body
                                        ##   in Loop: Header=BB0_1 Depth=1
	xorl	%eax, %eax              ## encoding: [0x31,0xc0]
	callq	_ftest                  ## encoding: [0xe8,A,A,A,A]
                                        ##   fixup A - offset: 1, value: _ftest-4, kind: reloc_branch_4byte_pcrel
	decl	%ebx                    ## encoding: [0xff,0xcb]
	jmp	LBB0_1                  ## encoding: [0xeb,A]
                                        ##   fixup A - offset: 1, value: LBB0_1-1, kind: FK_PCRel_1
LBB0_2:                                 ## %for.cond.cleanup
	addq	$8, %rsp                ## encoding: [0x48,0x83,0xc4,0x08]
	popq	%rbx                    ## encoding: [0x5b]
	popq	%rbp                    ## encoding: [0x5d]
	retq                            ## encoding: [0xc3]
	.cfi_endproc
                                        ## -- End function

The resulting loop body code for -Oz is longer than -Os. This is because the exit loop comparison is performed at the beginning of the loop resulting in an additional jump instruction and missing opportunity to fold the exit condition with the iv decrement.

I believe this is a regression from Version 7.0, as I don’t recall having seen this previously.  This affects most targets.

Should I create a bug report for that?

Thanks.

John

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