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[Codegenprepare][X86] Use usub with overflow opt for IV increment

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Function `replaceMathCmpWithIntrinsic` artificially limits the scope
of the optimization, setting a requirement of two instructions be in
the same block, due to two reasons:
- usage of DT for more general check is costly in terms of compile time;
- risk of creating a new value that lives through multiple blocks.

Because of this, two semantically equivalent tests may be or not be the
subject of this opt depending on where the binary operation is located.
See `test/CodeGen/X86/usub_inc_iv.ll` for motivation

There is one important particular case where this limitation is  too strict:
it is when the binary operation is the increment of the induction variable.
As result, the application of this opt becomes fragile and highly reliant on
where other passes decide to place IV increment. In most cases, they place
it in the end of the latch block, killing the opt opportunity (when in fact it
does not matter where to insert the actual instruction).

This patch handles this particular case separately.
- The detector does not use dom tree and has constant cost;
- The value of IV or IV.next lives through all loop in any case, so this should not
  create a new unexpected long-living value.

As result, the transform becomes more robust. It also seems to lead to
better code generation in some cases (see `test/CodeGen/X86/lsr-loop-exit-cond.ll`).

Differential Revision: https://reviews.llvm.org/D96119
Reviewed By: spatel, reames
This commit is contained in:
Max Kazantsev 2021-02-11 11:57:23 +07:00
parent a8eb60430b
commit 956eb929fb
4 changed files with 68 additions and 38 deletions
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33
lib/CodeGen/CodeGenPrepare.cpp
View File

@ -1284,7 +1284,29 @@ bool CodeGenPrepare::replaceMathCmpWithIntrinsic(BinaryOperator *BO,
Value *Arg0, Value *Arg1,
CmpInst *Cmp,
Intrinsic::ID IID) {
if (BO->getParent() != Cmp->getParent()) {
auto isIVIncrement = [this, &Cmp](BinaryOperator *BO) {
auto *PN = dyn_cast<PHINode>(BO->getOperand(0));
if (!PN)
return false;
const Loop *L = LI->getLoopFor(BO->getParent());
if (!L || L->getHeader() != PN->getParent() || !L->getLoopLatch())
return false;
if (PN->getIncomingValueForBlock(L->getLoopLatch()) != BO)
return false;
if (auto *Step = dyn_cast<Instruction>(BO->getOperand(1)))
if (L->contains(Step->getParent()))
return false;
// IV increment may have other users than the IV. We do not want to make
// dominance queries to analyze the legality of moving it towards the cmp,
// so just check that there is no other users.
if (!BO->hasOneUse())
return false;
// Do not risk on moving increment into a child loop.
if (LI->getLoopFor(Cmp->getParent()) != L)
return false;
return true;
};
if (BO->getParent() != Cmp->getParent() && !isIVIncrement(BO)) {
// We used to use a dominator tree here to allow multi-block optimization.
// But that was problematic because:
// 1. It could cause a perf regression by hoisting the math op into the
@ -1295,9 +1317,16 @@ bool CodeGenPrepare::replaceMathCmpWithIntrinsic(BinaryOperator *BO,
// This is because we recompute the DT on every change in the main CGP
// run-loop. The recomputing is probably unnecessary in many cases, so if
// that was fixed, using a DT here would be ok.
//
// There is one important particular case we still want to handle: if BO is
// the IV increment. Important properties that make it profitable:
// - We can speculate IV increment anywhere in the loop (as long as the
// indvar Phi is its only user);
// - Upon computing Cmp, we effectively compute something equivalent to the
// IV increment (despite it loops differently in the IR). So moving it up
// to the cmp point does not really increase register pressure.
return false;
}
// We allow matching the canonical IR (add X, C) back to (usubo X, -C).
if (BO->getOpcode() == Instruction::Add &&
IID == Intrinsic::usub_with_overflow) {

14
test/CodeGen/X86/2020_12_02_decrementing_loop.ll
View File

@ -89,15 +89,16 @@ failure: ; preds = %backedge
define i32 @test_02(i32* %p, i64 %len, i32 %x) {
; CHECK-LABEL: test_02:
; CHECK: ## %bb.0: ## %entry
; CHECK-NEXT: movq %rsi, %rax
; CHECK-NEXT: .p2align 4, 0x90
; CHECK-NEXT: LBB2_1: ## %loop
; CHECK-NEXT: ## =>This Inner Loop Header: Depth=1
; CHECK-NEXT: testq %rsi, %rsi
; CHECK-NEXT: je LBB2_4
; CHECK-NEXT: subq $1, %rax
; CHECK-NEXT: jb LBB2_4
; CHECK-NEXT: ## %bb.2: ## %backedge
; CHECK-NEXT: ## in Loop: Header=BB2_1 Depth=1
; CHECK-NEXT: cmpl %edx, -4(%rdi,%rsi,4)
; CHECK-NEXT: leaq -1(%rsi), %rsi
; CHECK-NEXT: movq %rax, %rsi
; CHECK-NEXT: jne LBB2_1
; CHECK-NEXT: ## %bb.3: ## %failure
; CHECK-NEXT: ud2
@ -132,15 +133,16 @@ failure: ; preds = %backedge
define i32 @test_03(i32* %p, i64 %len, i32 %x) {
; CHECK-LABEL: test_03:
; CHECK: ## %bb.0: ## %entry
; CHECK-NEXT: movq %rsi, %rax
; CHECK-NEXT: .p2align 4, 0x90
; CHECK-NEXT: LBB3_1: ## %loop
; CHECK-NEXT: ## =>This Inner Loop Header: Depth=1
; CHECK-NEXT: testq %rsi, %rsi
; CHECK-NEXT: je LBB3_4
; CHECK-NEXT: subq $1, %rax
; CHECK-NEXT: jb LBB3_4
; CHECK-NEXT: ## %bb.2: ## %backedge
; CHECK-NEXT: ## in Loop: Header=BB3_1 Depth=1
; CHECK-NEXT: cmpl %edx, -4(%rdi,%rsi,4)
; CHECK-NEXT: leaq -1(%rsi), %rsi
; CHECK-NEXT: movq %rax, %rsi
; CHECK-NEXT: jne LBB3_1
; CHECK-NEXT: ## %bb.3: ## %failure
; CHECK-NEXT: ud2

42
test/CodeGen/X86/lsr-loop-exit-cond.ll
View File

@ -16,11 +16,11 @@ define void @t(i8* nocapture %in, i8* nocapture %out, i32* nocapture %rk, i32 %r
; GENERIC-NEXT: movl (%rdx), %eax
; GENERIC-NEXT: movl 4(%rdx), %ebx
; GENERIC-NEXT: decl %ecx
; GENERIC-NEXT: leaq 20(%rdx), %r14
; GENERIC-NEXT: leaq 20(%rdx), %r11
; GENERIC-NEXT: movq _Te0@{{.*}}(%rip), %r9
; GENERIC-NEXT: movq _Te1@{{.*}}(%rip), %r8
; GENERIC-NEXT: movq _Te3@{{.*}}(%rip), %r10
; GENERIC-NEXT: movq %rcx, %r11
; GENERIC-NEXT: movq %rcx, %r14
; GENERIC-NEXT: .p2align 4, 0x90
; GENERIC-NEXT: LBB0_1: ## %bb
; GENERIC-NEXT: ## =>This Inner Loop Header: Depth=1
@ -32,30 +32,29 @@ define void @t(i8* nocapture %in, i8* nocapture %out, i32* nocapture %rk, i32 %r
; GENERIC-NEXT: movzbl %bpl, %ebp
; GENERIC-NEXT: movl (%r8,%rbp,4), %ebp
; GENERIC-NEXT: xorl (%r9,%rax,4), %ebp
; GENERIC-NEXT: xorl -12(%r14), %ebp
; GENERIC-NEXT: xorl -12(%r11), %ebp
; GENERIC-NEXT: shrl $24, %ebx
; GENERIC-NEXT: movl (%r10,%rdi,4), %edi
; GENERIC-NEXT: xorl (%r9,%rbx,4), %edi
; GENERIC-NEXT: xorl -8(%r14), %edi
; GENERIC-NEXT: xorl -8(%r11), %edi
; GENERIC-NEXT: movl %ebp, %eax
; GENERIC-NEXT: shrl $24, %eax
; GENERIC-NEXT: movl (%r9,%rax,4), %eax
; GENERIC-NEXT: testq %r11, %r11
; GENERIC-NEXT: je LBB0_3
; GENERIC-NEXT: subq $1, %r14
; GENERIC-NEXT: jb LBB0_3
; GENERIC-NEXT: ## %bb.2: ## %bb1
; GENERIC-NEXT: ## in Loop: Header=BB0_1 Depth=1
; GENERIC-NEXT: movl %edi, %ebx
; GENERIC-NEXT: shrl $16, %ebx
; GENERIC-NEXT: movzbl %bl, %ebx
; GENERIC-NEXT: xorl (%r8,%rbx,4), %eax
; GENERIC-NEXT: xorl -4(%r14), %eax
; GENERIC-NEXT: xorl -4(%r11), %eax
; GENERIC-NEXT: shrl $24, %edi
; GENERIC-NEXT: movzbl %bpl, %ebx
; GENERIC-NEXT: movl (%r10,%rbx,4), %ebx
; GENERIC-NEXT: xorl (%r9,%rdi,4), %ebx
; GENERIC-NEXT: xorl (%r14), %ebx
; GENERIC-NEXT: decq %r11
; GENERIC-NEXT: addq $16, %r14
; GENERIC-NEXT: xorl (%r11), %ebx
; GENERIC-NEXT: addq $16, %r11
; GENERIC-NEXT: jmp LBB0_1
; GENERIC-NEXT: LBB0_3: ## %bb2
; GENERIC-NEXT: shlq $4, %rcx
@ -99,12 +98,12 @@ define void @t(i8* nocapture %in, i8* nocapture %out, i32* nocapture %rk, i32 %r
; ATOM-NEXT: ## kill: def $ecx killed $ecx def $rcx
; ATOM-NEXT: movl (%rdx), %r15d
; ATOM-NEXT: movl 4(%rdx), %eax
; ATOM-NEXT: leaq 20(%rdx), %r14
; ATOM-NEXT: leaq 20(%rdx), %r11
; ATOM-NEXT: movq _Te0@{{.*}}(%rip), %r9
; ATOM-NEXT: movq _Te1@{{.*}}(%rip), %r8
; ATOM-NEXT: movq _Te3@{{.*}}(%rip), %r10
; ATOM-NEXT: decl %ecx
; ATOM-NEXT: movq %rcx, %r11
; ATOM-NEXT: movq %rcx, %r14
; ATOM-NEXT: .p2align 4, 0x90
; ATOM-NEXT: LBB0_1: ## %bb
; ATOM-NEXT: ## =>This Inner Loop Header: Depth=1
@ -118,28 +117,27 @@ define void @t(i8* nocapture %in, i8* nocapture %out, i32* nocapture %rk, i32 %r
; ATOM-NEXT: movzbl %r15b, %edi
; ATOM-NEXT: xorl (%r9,%rbp,4), %ebx
; ATOM-NEXT: movl (%r10,%rdi,4), %edi
; ATOM-NEXT: xorl -12(%r14), %ebx
; ATOM-NEXT: xorl -12(%r11), %ebx
; ATOM-NEXT: xorl (%r9,%rax,4), %edi
; ATOM-NEXT: movl %ebx, %eax
; ATOM-NEXT: xorl -8(%r14), %edi
; ATOM-NEXT: xorl -8(%r11), %edi
; ATOM-NEXT: shrl $24, %eax
; ATOM-NEXT: movl (%r9,%rax,4), %r15d
; ATOM-NEXT: testq %r11, %r11
; ATOM-NEXT: subq $1, %r14
; ATOM-NEXT: movl %edi, %eax
; ATOM-NEXT: je LBB0_3
; ATOM-NEXT: jb LBB0_3
; ATOM-NEXT: ## %bb.2: ## %bb1
; ATOM-NEXT: ## in Loop: Header=BB0_1 Depth=1
; ATOM-NEXT: shrl $16, %eax
; ATOM-NEXT: shrl $24, %edi
; ATOM-NEXT: decq %r11
; ATOM-NEXT: movzbl %al, %ebp
; ATOM-NEXT: movzbl %al, %eax
; ATOM-NEXT: xorl (%r8,%rax,4), %r15d
; ATOM-NEXT: movzbl %bl, %eax
; ATOM-NEXT: movl (%r10,%rax,4), %eax
; ATOM-NEXT: xorl (%r8,%rbp,4), %r15d
; ATOM-NEXT: xorl -4(%r11), %r15d
; ATOM-NEXT: xorl (%r9,%rdi,4), %eax
; ATOM-NEXT: xorl -4(%r14), %r15d
; ATOM-NEXT: xorl (%r14), %eax
; ATOM-NEXT: addq $16, %r14
; ATOM-NEXT: xorl (%r11), %eax
; ATOM-NEXT: addq $16, %r11
; ATOM-NEXT: jmp LBB0_1
; ATOM-NEXT: LBB0_3: ## %bb2
; ATOM-NEXT: shrl $16, %eax

17
test/CodeGen/X86/usub_inc_iv.ll
View File

@ -102,18 +102,19 @@ define i32 @test_02(i32* %p, i64 %len, i32 %x) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ], [ [[LEN:%.*]], [[ENTRY:%.*]] ]
; CHECK-NEXT: [[COND_1:%.*]] = icmp eq i64 [[IV]], 0
; CHECK-NEXT: br i1 [[COND_1]], label [[EXIT:%.*]], label [[BACKEDGE]]
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[MATH:%.*]], [[BACKEDGE:%.*]] ], [ [[LEN:%.*]], [[ENTRY:%.*]] ]
; CHECK-NEXT: [[TMP0:%.*]] = call { i64, i1 } @llvm.usub.with.overflow.i64(i64 [[IV]], i64 1)
; CHECK-NEXT: [[MATH]] = extractvalue { i64, i1 } [[TMP0]], 0
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i64, i1 } [[TMP0]], 1
; CHECK-NEXT: br i1 [[OV]], label [[EXIT:%.*]], label [[BACKEDGE]]
; CHECK: backedge:
; CHECK-NEXT: [[SUNKADDR:%.*]] = mul i64 [[IV]], 4
; CHECK-NEXT: [[TMP0:%.*]] = bitcast i32* [[P:%.*]] to i8*
; CHECK-NEXT: [[SUNKADDR1:%.*]] = getelementptr i8, i8* [[TMP0]], i64 [[SUNKADDR]]
; CHECK-NEXT: [[TMP1:%.*]] = bitcast i32* [[P:%.*]] to i8*
; CHECK-NEXT: [[SUNKADDR1:%.*]] = getelementptr i8, i8* [[TMP1]], i64 [[SUNKADDR]]
; CHECK-NEXT: [[SUNKADDR2:%.*]] = getelementptr i8, i8* [[SUNKADDR1]], i64 -4
; CHECK-NEXT: [[TMP1:%.*]] = bitcast i8* [[SUNKADDR2]] to i32*
; CHECK-NEXT: [[LOADED:%.*]] = load atomic i32, i32* [[TMP1]] unordered, align 4
; CHECK-NEXT: [[TMP2:%.*]] = bitcast i8* [[SUNKADDR2]] to i32*
; CHECK-NEXT: [[LOADED:%.*]] = load atomic i32, i32* [[TMP2]] unordered, align 4
; CHECK-NEXT: [[COND_2:%.*]] = icmp eq i32 [[LOADED]], [[X:%.*]]
; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], -1
; CHECK-NEXT: br i1 [[COND_2]], label [[FAILURE:%.*]], label [[LOOP]]
; CHECK: exit:
; CHECK-NEXT: ret i32 -1