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[SimplifyCFG] Fold `llvm.guard(false)` to unreachable 

Summary:
`llvm.guard(false)` always bails out of the current compilation unit, so
we can prune any control flow following it.

Reviewers: hfinkel, pcc, reames

Subscribers: majnemer, reames, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19245

llvm-svn: 266955
This commit is contained in:
Sanjoy Das 2016-04-21 05:09:12 +00:00
parent afd1b06d8e
commit 54a3a006ca
2 changed files with 105 additions and 1 deletions
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20
llvm/lib/Transforms/Utils/Local.cpp
View File

@ -1310,7 +1310,7 @@ static bool markAliveBlocks(Function &F,
// Assumptions that are known to be false are equivalent to unreachable.
// Also, if the condition is undefined, then we make the choice most
// beneficial to the optimizer, and choose that to also be unreachable.
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI)) {
if (II->getIntrinsicID() == Intrinsic::assume) {
bool MakeUnreachable = false;
if (isa<UndefValue>(II->getArgOperand(0)))
@ -1327,6 +1327,24 @@ static bool markAliveBlocks(Function &F,
}
}
if (II->getIntrinsicID() == Intrinsic::experimental_guard) {
// A call to the guard intrinsic bails out of the current compilation
// unit if the predicate passed to it is false. If the predicate is a
// constant false, then we know the guard will bail out of the current
// compile unconditionally, so all code following it is dead.
//
// Note: unlike in llvm.assume, it is not "obviously profitable" for
// guards to treat `undef` as `false` since a guard on `undef` can
// still be useful for widening.
if (auto *CI = dyn_cast<ConstantInt>(II->getArgOperand(0)))
if (CI->isZero() && !isa<UnreachableInst>(II->getNextNode())) {
changeToUnreachable(II->getNextNode(), /*UseLLVMTrap=*/ false);
Changed = true;
break;
}
}
}
if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
if (CI->doesNotReturn()) {
// If we found a call to a no-return function, insert an unreachable

86
llvm/test/Transforms/SimplifyCFG/guards.ll Normal file
View File

@ -0,0 +1,86 @@
; RUN: opt -S -simplifycfg < %s | FileCheck %s
declare void @llvm.experimental.guard(i1, ...)
define i32 @f_0(i1 %c) {
; CHECK-LABEL: @f_0(
; CHECK-NEXT: entry:
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
; CHECK-NEXT: unreachable
entry:
call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
ret i32 10
}
define i32 @f_1(i1 %c) {
; Demonstrate that we (intentionally) do not simplify a guard on undef
; CHECK-LABEL: @f_1(
; CHECK: ret i32 10
; CHECK: ret i32 20
entry:
br i1 %c, label %true, label %false
true:
call void(i1, ...) @llvm.experimental.guard(i1 undef) [ "deopt"() ]
ret i32 10
false:
ret i32 20
}
define i32 @f_2(i1 %c, i32* %buf) {
; CHECK-LABEL: @f_2(
entry:
br i1 %c, label %guard_block, label %merge_block
guard_block:
call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
%val = load i32, i32* %buf
br label %merge_block
merge_block:
%to.return = phi i32 [ %val, %guard_block ], [ 50, %entry ]
ret i32 %to.return
; CHECK: guard_block:
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
; CHECK-NEXT: unreachable
; CHECK: merge_block:
; CHECK-NEXT: ret i32 50
}
define i32 @f_3(i1* %c, i32* %buf) {
; CHECK-LABEL: @f_3(
entry:
%c0 = load volatile i1, i1* %c
br i1 %c0, label %guard_block, label %merge_block
guard_block:
call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
%val = load i32, i32* %buf
%c2 = load volatile i1, i1* %c
br i1 %c2, label %left, label %right
merge_block:
%c1 = load volatile i1, i1* %c
br i1 %c1, label %left, label %right
left:
%val.left = phi i32 [ %val, %guard_block ], [ 50, %merge_block ]
ret i32 %val.left
right:
%val.right = phi i32 [ %val, %guard_block ], [ 100, %merge_block ]
ret i32 %val.right
; CHECK: guard_block:
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
; CHECK-NEXT: unreachable
; CHECK: merge_block:
; CHECK-NEXT: %c1 = load volatile i1, i1* %c
; CHECK-NEXT: [[VAL:%[^ ]]] = select i1 %c1, i32 50, i32 100
; CHECK-NEXT: ret i32 [[VAL]]
}