[PATCH] D70965: [SelectionDAG] Expand nnan FMINNUM/FMAXNUM to select sequence

[Ulrich Weigand via Phabricator via llvm-commits]

uweigand created this revision.
uweigand added reviewers: spatel, cameron.mcinally, arsenm.
Herald added subscribers: llvm-commits, hiraditya, wdng.
Herald added a project: LLVM.

As discussed in D70852 <https://reviews.llvm.org/D70852>, currently LLVM may introduce calls to fmin/fmax into programs that originally did not have any dependency against libm, potentially causing failures at link time.

This patch attempts to solve this issue by adding code to TargetLowering::expandFMINNUM_FMAXNUM to expand FMINNUM/FMAXNUM to a compare+select sequence instead of the libcall.  This is done only if the node is marked as "nnan".  In this case, the expansion to compare+select is always correct.  This also catches all cases where the FMINNUM/FMAXNUM was synthesized by LLVM; this is also only done in the nnan case.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D70965

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/SystemZ/fp-libcall.ll


Index: llvm/test/CodeGen/SystemZ/fp-libcall.ll
===================================================================
--- llvm/test/CodeGen/SystemZ/fp-libcall.ll
+++ llvm/test/CodeGen/SystemZ/fp-libcall.ll
@@ -233,6 +233,68 @@
   ret fp128 %tmp
 }
 
+; Verify that "nnan" minnum/maxnum calls are transformed to
+; compare+select sequences instead of libcalls.
+define float @f34(float %x, float %y) {
+; CHECK-LABEL: f34:
+; CHECK: cebr %f0, %f2
+; CHECK: blr %r14
+; CHECK: ler %f0, %f2
+; CHECK: br %r14
+  %tmp = call nnan float @llvm.minnum.f32(float %x, float %y)
+  ret float %tmp
+}
+
+define double @f35(double %x, double %y) {
+; CHECK-LABEL: f35:
+; CHECK: cdbr %f0, %f2
+; CHECK: blr %r14
+; CHECK: ldr %f0, %f2
+; CHECK: br %r14
+  %tmp = call nnan double @llvm.minnum.f64(double %x, double %y)
+  ret double %tmp
+}
+
+define fp128 @f36(fp128 %x, fp128 %y) {
+; CHECK-LABEL: f36:
+; CHECK: cxbr
+; CHECK: jl
+; CHECK: lxr
+; CHECK: br %r14
+  %tmp = call nnan fp128 @llvm.minnum.f128(fp128 %x, fp128 %y)
+  ret fp128 %tmp
+}
+
+define float @f37(float %x, float %y) {
+; CHECK-LABEL: f37:
+; CHECK: cebr %f0, %f2
+; CHECK: bhr %r14
+; CHECK: ler %f0, %f2
+; CHECK: br %r14
+  %tmp = call nnan float @llvm.maxnum.f32(float %x, float %y)
+  ret float %tmp
+}
+
+define double @f38(double %x, double %y) {
+; CHECK-LABEL: f38:
+; CHECK: cdbr %f0, %f2
+; CHECK: bhr %r14
+; CHECK: ldr %f0, %f2
+; CHECK: br %r14
+  %tmp = call nnan double @llvm.maxnum.f64(double %x, double %y)
+  ret double %tmp
+}
+
+define fp128 @f39(fp128 %x, fp128 %y) {
+; CHECK-LABEL: f39:
+; CHECK: cxbr
+; CHECK: jh
+; CHECK: lxr
+; CHECK: br %r14
+  %tmp = call nnan fp128 @llvm.maxnum.f128(fp128 %x, fp128 %y)
+  ret fp128 %tmp
+}
+
 declare float @llvm.powi.f32(float, i32)
 declare double @llvm.powi.f64(double, i32)
 declare fp128 @llvm.powi.f128(fp128, i32)
Index: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -6225,6 +6225,22 @@
     }
   }
 
+  // If none of the above worked, but there are no NaNs, then expand to
+  // a compare/select sequence.  This is required for correctness since
+  // InstCombine might have canonicalized a fcmp+select sequence to a
+  // FMINNUM/FMAXNUM node.  If we were to fall through to the default
+  // expansion to libcall, we might introduce a link-time dependency
+  // on libm into a file that originally did not have one.
+  if (Node->getFlags().hasNoNaNs()) {
+    ISD::CondCode Pred =
+        Node->getOpcode() == ISD::FMINNUM ? ISD::SETLT : ISD::SETGT;
+    SDValue Tmp1 = Node->getOperand(0);
+    SDValue Tmp2 = Node->getOperand(1);
+    Tmp1 = DAG.getSelectCC(dl, Tmp1, Tmp2, Tmp1, Tmp2, Pred);
+    Tmp1->setFlags(Node->getFlags());
+    return Tmp1;
+  }
+
   return SDValue();
 }
 


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