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set div/rem default values to 'expensive' in TargetTransformInfo's cost model

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...because that's what the cost model was intended to do.

As discussed in D12882, this fix has a temporary unintended consequence for
SimplifyCFG: it causes us to not speculate an fdiv. However, two wrongs make
PR24818 right, and two wrongs make PR24343 act right even though it's really
still wrong.

I intend to correct SimplifyCFG and add to CodeGenPrepare to account for this
cost model change and preserve the righteousness for the bug report cases.

https://llvm.org/bugs/show_bug.cgi?id=24818
https://llvm.org/bugs/show_bug.cgi?id=24343

Differential Revision: http://reviews.llvm.org/D12882

llvm-svn: 248439
This commit is contained in:
Sanjay Patel 2015-09-23 22:28:18 +00:00
parent e2919f272c
commit 423048100e
2 changed files with 35 additions and 0 deletions
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8
include/llvm/Analysis/TargetTransformInfoImpl.h
View File

@ -61,6 +61,14 @@ public:
// Otherwise, the default basic cost is used.
return TTI::TCC_Basic;
case Instruction::FDiv:
case Instruction::FRem:
case Instruction::SDiv:
case Instruction::SRem:
case Instruction::UDiv:
case Instruction::URem:
return TTI::TCC_Expensive;
case Instruction::IntToPtr: {
// An inttoptr cast is free so long as the input is a legal integer type
// which doesn't contain values outside the range of a pointer.

27
test/Transforms/SimplifyCFG/speculate-math.ll
View File

@ -7,6 +7,33 @@ declare float @llvm.fabs.f32(float) nounwind readonly
declare float @llvm.minnum.f32(float, float) nounwind readonly
declare float @llvm.maxnum.f32(float, float) nounwind readonly
; FIXME: This is intended to be a temporary test. As discussed in
; D12882, we actually do want to speculate even expensive operations
; in SimplifyCFG because it can expose more optimizations for other
; passes. Therefore, we either need to adjust SimplifyCFG's
; calculations that use the TTI cost model or use a different cost
; model for deciding which ops should be speculated in SimplifyCFG.
; We should also be using the TTI cost model later - for example in
; CodeGenPrepare - to potentially undo this speculation.
; Do not speculate fdiv by default because it is generally expensive.
; CHECK-LABEL: @fdiv_test(
; CHECK-NOT: select
define double @fdiv_test(double %a, double %b) {
entry:
%cmp = fcmp ogt double %a, 0.0
br i1 %cmp, label %cond.true, label %cond.end
cond.true:
%div = fdiv double %b, %a
br label %cond.end
cond.end:
%cond = phi double [ %div, %cond.true ], [ 0.0, %entry ]
ret double %cond
}
; CHECK-LABEL: @sqrt_test(
; CHECK: select
define void @sqrt_test(float addrspace(1)* noalias nocapture %out, float %a) nounwind {