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.

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

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/speculate-math.ll b/llvm/test/Transforms/SimplifyCFG/speculate-math.ll
index 0ba93d29117..442e8677e97 100644
--- a/llvm/test/Transforms/SimplifyCFG/speculate-math.ll
+++ b/llvm/test/Transforms/SimplifyCFG/speculate-math.ll
@@ -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 {