[llvm] [Instcombine] Avoid widening trunc+sext to trunc+shl+ashr when not profitable (PR #160483)

[Wenju He via llvm-commits]

https://github.com/wenju-he created https://github.com/llvm/llvm-project/pull/160483

Skip the transform that replaces:
  %a = trunc i64 %x to i16
  %b = sext i16 %a to i32
with
  %a = trunc i64 %x to i32
  %b = shl i32 %a, 16
  %c = ashr exact i32 %b, 16
when (pre-trunc) source type is wider than the final destination type. Modern architectures typically have efficient sign-extend instruction. It is preferable to preserve the sext for this case.

Resolves #116019

>From e9d56808d3d686a1bde664353f6784af85c7be06 Mon Sep 17 00:00:00 2001
From: Wenju He <wenju.he at intel.com>
Date: Wed, 24 Sep 2025 12:09:26 +0200
Subject: [PATCH] [Instcombine] Avoid widening trunc+sext to trunc+shl+ashr
 when not profitable

Skip the transform that replaces:
  %a = trunc i64 %x to i16
  %b = sext i16 %a to i32
with
  %a = trunc i64 %x to i32
  %b = shl i32 %a, 16
  %c = ashr exact i32 %b, 16
when (pre-trunc) source type is wider than the final destination type.
Modern architectures typically have efficient sign-extend instruction.
It is preferable to preserve the sext for this case.

Resolves #116019
---
 .../Transforms/InstCombine/InstCombineCasts.cpp   | 15 ++++++++++++---
 llvm/test/Transforms/InstCombine/sext.ll          | 11 +++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 9ca8194b44f8f..ea029e47730bf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1528,7 +1528,17 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &Sext) {
   }
 
   // Try to extend the entire expression tree to the wide destination type.
-  if (shouldChangeType(SrcTy, DestTy) && canEvaluateSExtd(Src, DestTy)) {
+  bool shouldExtendExpression = true;
+  Value *X = nullptr;
+  // Do not extend expression in the trunc + sext pattern when destination type
+  // is narrower than original (pre-trunc) type: modern architectures typically
+  // provide efficient sign-extend instruction, so preserving the sext is
+  // preferable here.
+  if (match(Src, m_Trunc(m_Value(X))))
+    if (X->getType()->getScalarSizeInBits() > DestBitSize)
+      shouldExtendExpression = false;
+  if (shouldExtendExpression && shouldChangeType(SrcTy, DestTy) &&
+      canEvaluateSExtd(Src, DestTy)) {
     // Okay, we can transform this!  Insert the new expression now.
     LLVM_DEBUG(
         dbgs() << "ICE: EvaluateInDifferentType converting expression type"
@@ -1548,8 +1558,7 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &Sext) {
                                       ShAmt);
   }
 
-  Value *X;
-  if (match(Src, m_Trunc(m_Value(X)))) {
+  if (X) {
     // If the input has more sign bits than bits truncated, then convert
     // directly to final type.
     unsigned XBitSize = X->getType()->getScalarSizeInBits();
diff --git a/llvm/test/Transforms/InstCombine/sext.ll b/llvm/test/Transforms/InstCombine/sext.ll
index c72614d526036..92748034f52ab 100644
--- a/llvm/test/Transforms/InstCombine/sext.ll
+++ b/llvm/test/Transforms/InstCombine/sext.ll
@@ -320,6 +320,17 @@ define i10 @test19(i10 %i) {
   ret i10 %d
 }
 
+define i32 @test20(i64 %i) {
+; CHECK-LABEL: @test20(
+; CHECK-NEXT:    [[A:%.*]] = trunc i64 [[I:%.*]] to i16
+; CHECK-NEXT:    [[B:%.*]] = sext i16 [[A]] to i32
+; CHECK-NEXT:    ret i32 [[B]]
+;
+  %a = trunc i64 %i to i16
+  %b = sext i16 %a to i32
+  ret i32 %b
+}
+
 define i32 @smear_set_bit(i32 %x) {
 ; CHECK-LABEL: @smear_set_bit(
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl i32 [[X:%.*]], 24