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Second part of pr16069 

The problem this time seems to be a thinko. We were assuming that in the CFG

A
| \
|  B
| /
C

speculating the basic block B would cause only the phi value for the B->C edge
to be speculated. That is not true, the phi's are semantically in the edges, so
if the A->B->C path is taken, any code needed for A->C is not executed and we
have to consider it too when deciding to speculate B.

llvm-svn: 183226
This commit is contained in:
Rafael Espindola 2013-06-04 14:11:59 +00:00
parent 863746eb1a
commit a5e536ab0e
2 changed files with 24 additions and 5 deletions
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13
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -1537,18 +1537,23 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
Value *OrigV = PN->getIncomingValueForBlock(BB); Value *OrigV = PN->getIncomingValueForBlock(BB);
Value *ThenV = PN->getIncomingValueForBlock(ThenBB); Value *ThenV = PN->getIncomingValueForBlock(ThenBB);
// FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf.
// Skip PHIs which are trivial. // Skip PHIs which are trivial.
if (ThenV == OrigV) if (ThenV == OrigV)
continue; continue;
HaveRewritablePHIs = true; HaveRewritablePHIs = true;
ConstantExpr *CE = dyn_cast<ConstantExpr>(ThenV); ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
if (!CE) ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
if (!OrigCE && !ThenCE)
continue; // Known safe and cheap. continue; // Known safe and cheap.
if (!isSafeToSpeculativelyExecute(CE)) if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE)) ||
(OrigCE && !isSafeToSpeculativelyExecute(OrigCE)))
return false; return false;
if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold) unsigned OrigCost = OrigCE ? ComputeSpeculationCost(OrigCE) : 0;
unsigned ThenCost = ThenCE ? ComputeSpeculationCost(ThenCE) : 0;
if (OrigCost + ThenCost > 2 * PHINodeFoldingThreshold)
return false; return false;
// Account for the cost of an unfolded ConstantExpr which could end up // Account for the cost of an unfolded ConstantExpr which could end up

16
llvm/test/Transforms/SimplifyCFG/PR16069.ll
View File

@ -1,8 +1,9 @@
; RUN: opt < %s -simplifycfg -S | FileCheck %s ; RUN: opt < %s -simplifycfg -S | FileCheck %s
; CHECK-NOT: select
@b = extern_weak global i32 @b = extern_weak global i32
define i32 @foo(i1 %y) { define i32 @foo(i1 %y) {
; CHECK: define i32 @foo(i1 %y) {
br i1 %y, label %bb1, label %bb2 br i1 %y, label %bb1, label %bb2
bb1: bb1:
br label %bb3 br label %bb3
@ -10,5 +11,18 @@ bb2:
br label %bb3 br label %bb3
bb3: bb3:
%cond.i = phi i32 [ 0, %bb1 ], [ srem (i32 1, i32 zext (i1 icmp eq (i32* @b, i32* null) to i32)), %bb2 ] %cond.i = phi i32 [ 0, %bb1 ], [ srem (i32 1, i32 zext (i1 icmp eq (i32* @b, i32* null) to i32)), %bb2 ]
; CHECK: phi i32 {{.*}} srem (i32 1, i32 zext (i1 icmp eq (i32* @b, i32* null) to i32)), %bb2
ret i32 %cond.i ret i32 %cond.i
} }
define i32 @foo2(i1 %x) {
; CHECK: define i32 @foo2(i1 %x) {
bb0:
br i1 %x, label %bb1, label %bb2
bb1:
br label %bb2
bb2:
%cond = phi i32 [ 0, %bb1 ], [ srem (i32 1, i32 zext (i1 icmp eq (i32* @b, i32* null) to i32)), %bb0 ]
; CHECK: %cond = phi i32 [ 0, %bb1 ], [ srem (i32 1, i32 zext (i1 icmp eq (i32* @b, i32* null) to i32)), %bb0 ]
ret i32 %cond
}