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[CGP] Limit distance between overflow math and cmp 

Inserting an overflowing arithmetic intrinsic can increase register
pressure by producing two values at a point where only one is needed,
while the second use maybe several blocks away. This increase in
pressure is likely to be more detrimental on performance than
rematerialising one of the original instructions.
    
So, check that the arithmetic and compare instructions are no further
apart than their immediate successor/predecessor.

Differential Revision: https://reviews.llvm.org/D59024

llvm-svn: 355823
This commit is contained in:
Sam Parker 2019-03-11 13:19:46 +00:00
parent b60aea4131
commit 52760bf435
2 changed files with 67 additions and 0 deletions
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11
llvm/lib/CodeGen/CodeGenPrepare.cpp
View File

@ -1180,6 +1180,17 @@ static bool replaceMathCmpWithIntrinsic(BinaryOperator *BO, CmpInst *Cmp,
bool MathDominates = DT.dominates(BO, Cmp);
if (!MathDominates && !DT.dominates(Cmp, BO))
return false;
// Check that the insertion doesn't create a value that is live across more
// than two blocks, so to minimise the increase in register pressure.
if (BO->getParent() != Cmp->getParent()) {
BasicBlock *Dominator = MathDominates ? BO->getParent() : Cmp->getParent();
BasicBlock *Dominated = MathDominates ? Cmp->getParent() : BO->getParent();
auto Successors = successors(Dominator);
if (llvm::find(Successors, Dominated) == Successors.end())
return false;
}
InsertPt = MathDominates ? cast<Instruction>(BO) : cast<Instruction>(Cmp);
}

56
llvm/test/Transforms/CodeGenPrepare/ARM/overflow-intrinsics.ll Normal file
View File

@ -0,0 +1,56 @@
; RUN: opt -codegenprepare -S < %s | FileCheck %s
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv8m.main-arm-none-eabi"
; CHECK-LABEL: uadd_overflow_too_far_cmp_dom
; CHECK-NOT: with.overflow.i32
define i32 @uadd_overflow_too_far_cmp_dom(i32 %arg0) {
entry:
%cmp = icmp ne i32 %arg0, 0
br i1 %cmp, label %if.else, label %if.then
if.then:
call void @foo()
br label %exit
if.else:
call void @bar()
br label %if.end
if.end:
%dec = add nsw i32 %arg0, -1
br label %exit
exit:
%res = phi i32 [ %arg0, %if.then ], [ %dec, %if.end ]
ret i32 %res
}
; CHECK-LABEL: uadd_overflow_too_far_math_dom
; CHECK-NOT: with.overflow.i32
define i32 @uadd_overflow_too_far_math_dom(i32 %arg0, i32 %arg1) {
entry:
%dec = add nsw i32 %arg0, -1
%cmp = icmp ugt i32 %arg0, 1
br i1 %cmp, label %if.else, label %if.then
if.then:
call void @foo()
br label %if.end
if.else:
call void @bar()
br label %if.end
if.end:
%cmp.i.i = icmp ne i32 %arg0, 0
%tobool = zext i1 %cmp.i.i to i32
br label %exit
exit:
ret i32 %tobool
}
declare void @foo()
declare void @bar()