[llvm] r248439 - set div/rem default values to 'expensive' in TargetTransformInfo's cost model

Sanjay Patel via llvm-commits llvm-commits at lists.llvm.org
Wed Sep 23 15:28:18 PDT 2015

Author: spatel
Date: Wed Sep 23 17:28:18 2015
New Revision: 248439

URL: http://llvm.org/viewvc/llvm-project?rev=248439&view=rev
set div/rem default values to 'expensive' in TargetTransformInfo's cost model

...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.


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


Modified: llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h?rev=248439&r1=248438&r2=248439&view=diff
--- llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h (original)
+++ llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h Wed Sep 23 17:28:18 2015
@@ -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.

Modified: llvm/trunk/test/Transforms/SimplifyCFG/speculate-math.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/speculate-math.ll?rev=248439&r1=248438&r2=248439&view=diff
--- llvm/trunk/test/Transforms/SimplifyCFG/speculate-math.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/speculate-math.ll Wed Sep 23 17:28:18 2015
@@ -7,6 +7,33 @@ declare float @llvm.fabs.f32(float) noun
 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) {
+  %cmp = fcmp ogt double %a, 0.0
+  br i1 %cmp, label %cond.true, label %cond.end
+  %div = fdiv double %b, %a
+  br label %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 {

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