84 lines
3.1 KiB
Diff
84 lines
3.1 KiB
Diff
Pull in r248439 from upstream llvm trunk (by Sanjay Patel):
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set div/rem default values to 'expensive' in TargetTransformInfo's
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cost model
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...because that's what the cost model was intended to do.
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As discussed in D12882, this fix has a temporary unintended
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consequence for SimplifyCFG: it causes us to not speculate an fdiv.
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However, two wrongs make PR24818 right, and two wrongs make PR24343
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act right even though it's really still wrong.
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I intend to correct SimplifyCFG and add to CodeGenPrepare to account
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for this cost model change and preserve the righteousness for the bug
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report cases.
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https://llvm.org/bugs/show_bug.cgi?id=24818
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https://llvm.org/bugs/show_bug.cgi?id=24343
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Differential Revision: http://reviews.llvm.org/D12882
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This fixes the too-eager fdiv hoisting in pow(), which could lead to
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unexpected floating point exceptions.
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Introduced here: http://svnweb.freebsd.org/changeset/base/288195
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Index: include/llvm/Analysis/TargetTransformInfoImpl.h
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===================================================================
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--- include/llvm/Analysis/TargetTransformInfoImpl.h
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+++ include/llvm/Analysis/TargetTransformInfoImpl.h
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@@ -60,6 +60,14 @@ class TargetTransformInfoImplBase {
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// Otherwise, the default basic cost is used.
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return TTI::TCC_Basic;
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+ case Instruction::FDiv:
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+ case Instruction::FRem:
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+ case Instruction::SDiv:
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+ case Instruction::SRem:
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+ case Instruction::UDiv:
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+ case Instruction::URem:
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+ return TTI::TCC_Expensive;
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+
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case Instruction::IntToPtr: {
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// An inttoptr cast is free so long as the input is a legal integer type
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// which doesn't contain values outside the range of a pointer.
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Index: test/Transforms/SimplifyCFG/speculate-math.ll
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===================================================================
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--- test/Transforms/SimplifyCFG/speculate-math.ll
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+++ test/Transforms/SimplifyCFG/speculate-math.ll
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@@ -7,6 +7,33 @@ declare float @llvm.fabs.f32(float) nounwind reado
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declare float @llvm.minnum.f32(float, float) nounwind readonly
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declare float @llvm.maxnum.f32(float, float) nounwind readonly
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+; FIXME: This is intended to be a temporary test. As discussed in
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+; D12882, we actually do want to speculate even expensive operations
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+; in SimplifyCFG because it can expose more optimizations for other
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+; passes. Therefore, we either need to adjust SimplifyCFG's
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+; calculations that use the TTI cost model or use a different cost
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+; model for deciding which ops should be speculated in SimplifyCFG.
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+; We should also be using the TTI cost model later - for example in
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+; CodeGenPrepare - to potentially undo this speculation.
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+
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+; Do not speculate fdiv by default because it is generally expensive.
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+
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+; CHECK-LABEL: @fdiv_test(
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+; CHECK-NOT: select
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+define double @fdiv_test(double %a, double %b) {
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+entry:
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+ %cmp = fcmp ogt double %a, 0.0
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+ br i1 %cmp, label %cond.true, label %cond.end
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+
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+cond.true:
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+ %div = fdiv double %b, %a
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+ br label %cond.end
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+
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+cond.end:
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+ %cond = phi double [ %div, %cond.true ], [ 0.0, %entry ]
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+ ret double %cond
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+}
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+
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; CHECK-LABEL: @sqrt_test(
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; CHECK: select
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define void @sqrt_test(float addrspace(1)* noalias nocapture %out, float %a) nounwind {
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