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[x86] add/sub (X==0) --> sbb(neg X)

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Our handling of select-of-constants is lumpy in IR (https://reviews.llvm.org/D24480),
lumpy in DAGCombiner, and lumpy in X86ISelLowering. That's why we only had the 'sbb'
codegen in 1 out of the 4 tests. This is a step towards smoothing that out.

First, show that all of these IR forms are equivalent:
http://rise4fun.com/Alive/mx

Second, show that the 'sbb' version is faster/smaller. IACA output for SandyBridge
(later Intel and AMD chips are similar based on Agner's tables):

This is the "obvious" x86 codegen (what gcc appears to produce currently):

| Num Of |              Ports pressure in cycles               |    |
|  Uops  |  0  - DV  |  1  |  2  -  D  |  3  -  D  |  4  |  5  |    |
---------------------------------------------------------------------
|   1*   |           |     |           |           |     |     |    | xor eax, eax
|   1    | 1.0       |     |           |           |     |     | CP | test edi, edi
|   1    |           |     |           |           |     | 1.0 | CP | setnz al
|   1    |           | 1.0 |           |           |     |     | CP | neg eax


This is the adc version:
|   1*   |           |     |           |           |     |     |    | xor eax, eax
|   1    | 1.0       |     |           |           |     |     | CP | cmp edi, 0x1
|   2    |           | 1.0 |           |           |     | 1.0 | CP | adc eax, 0xffffffff


And this is sbb:
|   1    | 1.0       |     |           |           |     |     |    | neg edi
|   2    |           | 1.0 |           |           |     | 1.0 | CP | sbb eax, eax

If IACA is trustworthy, then sbb became a single uop in Broadwell, so this will be
clearly better than the alternatives going forward.

llvm-svn: 306040
This commit is contained in:
Sanjay Patel 2017-06-22 18:11:19 +00:00
parent 42aac12546
commit 0fe549ac91
2 changed files with 25 additions and 12 deletions
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22
lib/Target/X86/X86ISelLowering.cpp
View File

@ -34897,13 +34897,29 @@ static SDValue combineAddOrSubToADCOrSBB(SDNode *N, SelectionDAG &DAG) {
!Cmp.getOperand(0).getValueType().isInteger())
return SDValue();
// (cmp Z, 1) sets the carry flag if Z is 0.
SDValue Z = Cmp.getOperand(0);
SDVTList VTs = DAG.getVTList(N->getValueType(0), MVT::i32);
// If X is -1 or 0, then we have an opportunity to avoid constants required by
// the cmp transform below. 'neg' sets the carry flag when Z != 0, so create 0
// or -1 using 'sbb' with fake operands:
// 0 - (Z != 0) --> sbb %eax, %eax, (neg Z)
// -1 + (Z == 0) --> sbb %eax, %eax, (neg Z)
if (auto *ConstantX = dyn_cast<ConstantSDNode>(X)) {
if ((IsSub && CC == X86::COND_NE && ConstantX->isNullValue()) ||
(!IsSub && CC == X86::COND_E && ConstantX->isAllOnesValue())) {
SDValue Zero = DAG.getConstant(0, DL, VT);
SDValue Neg = DAG.getNode(X86ISD::SUB, DL, VTs, Zero, Z);
return DAG.getNode(X86ISD::SETCC_CARRY, DL, VT,
DAG.getConstant(X86::COND_B, DL, MVT::i8),
SDValue(Neg.getNode(), 1));
}
}
// (cmp Z, 1) sets the carry flag if Z is 0.
SDValue NewCmp = DAG.getNode(X86ISD::CMP, DL, MVT::i32, Z,
DAG.getConstant(1, DL, Z.getValueType()));
SDVTList VTs = DAG.getVTList(N->getValueType(0), MVT::i32);
// X - (Z != 0) --> sub X, (zext(setne Z, 0)) --> adc X, -1, (cmp Z, 1)
// X + (Z != 0) --> add X, (zext(setne Z, 0)) --> sbb X, -1, (cmp Z, 1)
if (CC == X86::COND_NE)

15
test/CodeGen/X86/sbb.ll
View File

@ -8,9 +8,8 @@
define i8 @i8_select_0_or_neg1(i8 %x) {
; CHECK-LABEL: i8_select_0_or_neg1:
; CHECK: # BB#0:
; CHECK-NEXT: cmpb $1, %dil
; CHECK-NEXT: movb $-1, %al
; CHECK-NEXT: adcb $0, %al
; CHECK-NEXT: negb %dil
; CHECK-NEXT: sbbb %al, %al
; CHECK-NEXT: retq
%cmp = icmp eq i8 %x, 0
%sel = select i1 %cmp, i8 0, i8 -1
@ -22,9 +21,8 @@ define i8 @i8_select_0_or_neg1(i8 %x) {
define i16 @i16_select_0_or_neg1_as_math(i16 %x) {
; CHECK-LABEL: i16_select_0_or_neg1_as_math:
; CHECK: # BB#0:
; CHECK-NEXT: cmpw $1, %di
; CHECK-NEXT: movw $-1, %ax
; CHECK-NEXT: adcw $0, %ax
; CHECK-NEXT: negw %di
; CHECK-NEXT: sbbw %ax, %ax
; CHECK-NEXT: retq
%cmp = icmp eq i16 %x, 0
%ext = zext i1 %cmp to i16
@ -50,9 +48,8 @@ define i32 @i32_select_0_or_neg1_commuted(i32 %x) {
define i64 @i64_select_0_or_neg1_commuted_as_math(i64 %x) {
; CHECK-LABEL: i64_select_0_or_neg1_commuted_as_math:
; CHECK: # BB#0:
; CHECK-NEXT: xorl %eax, %eax
; CHECK-NEXT: cmpq $1, %rdi
; CHECK-NEXT: adcq $-1, %rax
; CHECK-NEXT: negq %rdi
; CHECK-NEXT: sbbq %rax, %rax
; CHECK-NEXT: retq
%cmp = icmp ne i64 %x, 0
%ext = zext i1 %cmp to i64