[llvm] [InstCombine] Handle scalable splats of constants in getMinimumFPType (PR #132960)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

We previously handled ConstantExpr scalable splats in 5d929794a87602cfd873381e11cc99149196bb49, but only fpexts.

ConstantExpr fpexts have since been removed, and simultaneously we didn't handle splats of constants that weren't extended.

This updates it to remove the fpext check and instead see if we can shrink the result of getSplatValue.

Note that the test case doesn't get completely folded away due to #<!-- -->132922




---
Full diff: https://github.com/llvm/llvm-project/pull/132960.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+6-5) 
- (modified) llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll (+13) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 4ec1af394464b..3faaf1e52db26 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1685,11 +1685,12 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
       return T;
 
   // We can only correctly find a minimum type for a scalable vector when it is
-  // a splat. For splats of constant values the fpext is wrapped up as a
-  // ConstantExpr.
-  if (auto *FPCExt = dyn_cast<ConstantExpr>(V))
-    if (FPCExt->getOpcode() == Instruction::FPExt)
-      return FPCExt->getOperand(0)->getType();
+  // a splat.
+  if (auto *FPCE = dyn_cast<ConstantExpr>(V))
+    if (isa<ScalableVectorType>(V->getType()))
+      if (auto *Splat = dyn_cast<ConstantFP>(FPCE->getSplatValue()))
+        if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
+          return T;
 
   // Try to shrink a vector of FP constants. This returns nullptr on scalable
   // vectors
diff --git a/llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll b/llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll
index 731b079881f08..595486361d16e 100644
--- a/llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll
+++ b/llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll
@@ -13,3 +13,16 @@ define <vscale x 2 x float> @shrink_splat_scalable_extend(<vscale x 2 x float> %
   %5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
   ret <vscale x 2 x float> %5
 }
+
+define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(<vscale x 2 x float> %a) {
+; CHECK-LABEL: define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(
+; CHECK-SAME: <vscale x 2 x float> [[A:%.*]]) {
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc <vscale x 2 x double> splat (double -1.000000e+00) to <vscale x 2 x float>
+; CHECK-NEXT:    [[TMP3:%.*]] = fadd <vscale x 2 x float> [[A]], [[TMP1]]
+; CHECK-NEXT:    ret <vscale x 2 x float> [[TMP3]]
+;
+  %2 = fpext <vscale x 2 x float> %a to <vscale x 2 x double>
+  %4 = fadd <vscale x 2 x double> %2, splat (double -1.000000e+00)
+  %5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
+  ret <vscale x 2 x float> %5
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/132960