[IR] allow fast-math-flags on phi of FP values (2nd try)

The changes here are based on the corresponding diffs for allowing FMF on 'select':
D61917 <https://reviews.llvm.org/D61917>

As discussed there, we want to have fast-math-flags be a property of an FP value
because the alternative (having them on things like fcmp) leads to logical
inconsistency such as:
https://bugs.llvm.org/show_bug.cgi?id=38086

The earlier patch for select made almost no practical difference because most
unoptimized conditional code begins life as a phi (based on what I see in clang).
Similarly, I don't expect this patch to do much on its own either because
SimplifyCFG promptly drops the flags when converting to select on a minimal
example like:
https://bugs.llvm.org/show_bug.cgi?id=39535

But once we have this plumbing in place, we should be able to wire up the FMF
propagation and start solving cases like that.

The change to RecurrenceDescriptor::AddReductionVar() is required to prevent a
regression in a LoopVectorize test. We are intersecting the FMF of any
FPMathOperator there, so if a phi is not properly annotated, new math
instructions may not be either. Once we fix the propagation in SimplifyCFG, it
may be safe to remove that hack.

Differential Revision: https://reviews.llvm.org/D67564

llvm-svn: 372878

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/test/Bitcode/compatibility.ll b/llvm/test/Bitcode/compatibility.ll
index b17aeda8ebb..9e7f6ac9917 100644
--- a/llvm/test/Bitcode/compatibility.ll
+++ b/llvm/test/Bitcode/compatibility.ll
@@ -861,6 +861,27 @@ define void @fastmathflags_vector_select(<2 x i1> %cond, <2 x double> %op1, <2 x
   ret void
 }
 
+define void @fastmathflags_phi(i1 %cond, float %f1, float %f2, double %d1, double %d2, half %h1, half %h2) {
+entry:
+  br i1 %cond, label %L1, label %L2
+L1:
+  br label %exit
+L2:
+  br label %exit
+exit:
+  %p.nnan = phi nnan float [ %f1, %L1 ], [ %f2, %L2 ]
+  ; CHECK: %p.nnan = phi nnan float [ %f1, %L1 ], [ %f2, %L2 ]
+  %p.ninf = phi ninf double [ %d1, %L1 ], [ %d2, %L2 ]
+  ; CHECK: %p.ninf = phi ninf double [ %d1, %L1 ], [ %d2, %L2 ]
+  %p.contract = phi contract half [ %h1, %L1 ], [ %h2, %L2 ]
+  ; CHECK: %p.contract = phi contract half [ %h1, %L1 ], [ %h2, %L2 ]
+  %p.nsz.reassoc = phi reassoc nsz float [ %f1, %L1 ], [ %f2, %L2 ]
+  ; CHECK: %p.nsz.reassoc = phi reassoc nsz float [ %f1, %L1 ], [ %f2, %L2 ]
+  %p.fast = phi fast half [ %h2, %L1 ], [ %h1, %L2 ]
+  ; CHECK: %p.fast = phi fast half [ %h2, %L1 ], [ %h1, %L2 ]
+  ret void
+}
+
 ; Check various fast math flags and floating-point types on calls.
 
 declare float @fmf1()