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EarlyCSE should ignore calls to @llvm.assume

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EarlyCSE uses a simple generation scheme for handling memory-based
dependencies, and calls to @llvm.assume (which are marked as writing to memory
to ensure the preservation of control dependencies) disturb that scheme
unnecessarily. Skipping calls to @llvm.assume is legal, and the alternative
(adding AA calls in EarlyCSE) is likely undesirable (we have GVN for that).

Fixes PR21448.

llvm-svn: 221175
This commit is contained in:
Hal Finkel 2014-11-03 20:21:32 +00:00
parent 5b15714b76
commit d731cfc8f4
2 changed files with 48 additions and 0 deletions
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12
lib/Transforms/Scalar/EarlyCSE.cpp
View File

@ -21,6 +21,8 @@
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
@ -28,6 +30,7 @@
#include "llvm/Transforms/Utils/Local.h"
#include <deque>
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "early-cse"
@ -435,6 +438,15 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
continue;
}
// Skip assume intrinsics, they don't really have side effects (although
// they're marked as such to ensure preservation of control dependencies),
// and this pass will not disturb any of the assumption's control
// dependencies.
if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
continue;
}
// If the instruction can be simplified (e.g. X+0 = X) then replace it with
// its simpler value.
if (Value *V = SimplifyInstruction(Inst, DL, TLI, DT, AT)) {

36
test/Transforms/EarlyCSE/basic.ll
View File

@ -1,5 +1,6 @@
; RUN: opt < %s -S -early-cse | FileCheck %s
declare void @llvm.assume(i1) nounwind
; CHECK-LABEL: @test1(
define void @test1(i8 %V, i32 *%P) {
@ -42,6 +43,16 @@ define i32 @test2(i32 *%P) {
; CHECK: ret i32 0
}
; CHECK-LABEL: @test2a(
define i32 @test2a(i32 *%P, i1 %b) {
%V1 = load i32* %P
tail call void @llvm.assume(i1 %b)
%V2 = load i32* %P
%Diff = sub i32 %V1, %V2
ret i32 %Diff
; CHECK: ret i32 0
}
;; Cross block load value numbering.
; CHECK-LABEL: @test3(
define i32 @test3(i32 *%P, i1 %Cond) {
@ -58,6 +69,22 @@ F:
; CHECK: ret i32 0
}
; CHECK-LABEL: @test3a(
define i32 @test3a(i32 *%P, i1 %Cond, i1 %b) {
%V1 = load i32* %P
br i1 %Cond, label %T, label %F
T:
store i32 4, i32* %P
ret i32 42
F:
tail call void @llvm.assume(i1 %b)
%V2 = load i32* %P
%Diff = sub i32 %V1, %V2
ret i32 %Diff
; CHECK: F:
; CHECK: ret i32 0
}
;; Cross block load value numbering stops when stores happen.
; CHECK-LABEL: @test4(
define i32 @test4(i32 *%P, i1 %Cond) {
@ -97,6 +124,15 @@ define i32 @test6(i32 *%P) {
; CHECK: ret i32 42
}
; CHECK-LABEL: @test6a(
define i32 @test6a(i32 *%P, i1 %b) {
store i32 42, i32* %P
tail call void @llvm.assume(i1 %b)
%V1 = load i32* %P
ret i32 %V1
; CHECK: ret i32 42
}
;; Trivial dead store elimination.
; CHECK-LABEL: @test7(
define void @test7(i32 *%P) {