[llvm] [X86] Explicitly widen larger than v4f16 to the legal v8f16 (NFC) (PR #153839)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Adam Nemet (anemet)

<details>
<summary>Changes</summary>

This patch makes the current behavior explicit to prepare for adding VTs for v[567]f16.

Right now these types are EVTs and hence don't fall under getPreferredVectorAction and are simply widened to the next legal power-of-two vector type.  For SSE2 this is v8f16.

Without the preparatory patch however, the behavior would change after adding these types.  getPreferredVectorAction would try to split them because this is the current behavior for any f16 vector type that is not legal.

There is a lot more detail at
https://github.com/llvm/llvm-project/issues/152150 in particular how splitting these new types leads to an inconsistency between NumRegistersForVT and getTypeAction.

The patch ensures that after the new types are added they would continue to be widened rather than split.  Once the patch to enable v[567]f16 lands, it will be an NFC for x86.

---
Full diff: https://github.com/llvm/llvm-project/pull/153839.diff


2 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+3-1) 
- (added) llvm/test/CodeGen/X86/v7f16-crash.ll (+14) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 97cdf5b784bc0..98a4f3556069a 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2756,8 +2756,10 @@ X86TargetLowering::getPreferredVectorAction(MVT VT) const {
       !Subtarget.hasBWI())
     return TypeSplitVector;
 
+  // Since v8f16 is legal, widen anything over v4f16.
   if (!VT.isScalableVector() && VT.getVectorNumElements() != 1 &&
-      !Subtarget.hasF16C() && VT.getVectorElementType() == MVT::f16)
+      VT.getVectorNumElements() <= 4 && !Subtarget.hasF16C() &&
+      VT.getVectorElementType() == MVT::f16)
     return TypeSplitVector;
 
   if (!VT.isScalableVector() && VT.getVectorNumElements() != 1 &&
diff --git a/llvm/test/CodeGen/X86/v7f16-crash.ll b/llvm/test/CodeGen/X86/v7f16-crash.ll
new file mode 100644
index 0000000000000..6db3e555028cc
--- /dev/null
+++ b/llvm/test/CodeGen/X86/v7f16-crash.ll
@@ -0,0 +1,14 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown-eabi-elf | FileCheck %s
+
+; CHECK-LABEL: conv2d
+define dso_local void @conv2d() {
+.preheader:
+  br label %0
+
+0:                                                ; preds = %0, %.preheader
+  %1 = phi [4 x <7 x half>] [ zeroinitializer, %.preheader ], [ %4, %0 ]
+  %2 = extractvalue [4 x <7 x half>] %1, 0
+  %3 = extractvalue [4 x <7 x half>] %1, 1
+  %4 = insertvalue [4 x <7 x half>] poison, <7 x half> poison, 3
+  br label %0
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/153839