[llvm] [InstCombine] Use common flags for shl in shl/trunc fold. (PR #221769)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/221769

As pointed out post-commit in
https://github.com/llvm/llvm-project/pull/219443, whether the flags from
the trunc can be transferred directly to the shl depends on the involved
types: https://alive2.llvm.org/ce/z/zN_TFq

For now, just use the common flags.

>From 91a961ed5449f7592fd7bb21108c2d9e46674003 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 7 Sep 2026 16:46:38 +0100
Subject: [PATCH 1/2] [InstCombine] Add test for incorrectly preserving flags.

---
 llvm/test/Transforms/InstCombine/trunc.ll | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index 59c0f83b0e60c..868f3a7ebd666 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -708,6 +708,17 @@ define i32 @trunc_nuw_shl_nsw_3_i32_i64(i64 %val) {
   ret i32 %trunc
 }
 
+define i4 @trunc_nuw_shl_i6_to_i4(i6 %x, i6 %y) {
+; CHECK-LABEL: @trunc_nuw_shl_i6_to_i4(
+; CHECK-NEXT:    [[X_TR:%.*]] = trunc i6 [[X:%.*]] to i4
+; CHECK-NEXT:    [[TRUNC:%.*]] = shl nuw i4 [[X_TR]], 3
+; CHECK-NEXT:    ret i4 [[TRUNC]]
+;
+  %shl = shl i6 %x, 3
+  %trunc = trunc nuw i6 %shl to i4
+  ret i4 %trunc
+}
+
 define i16 @trunc_shl_15_i16_i64(i64 %val) {
 ; CHECK-LABEL: @trunc_shl_15_i16_i64(
 ; CHECK-NEXT:    [[VAL_TR:%.*]] = trunc i64 [[VAL:%.*]] to i16
@@ -1553,3 +1564,4 @@ define i1 @neg_trunc_nuw_lshr(i8 %x, i8 %c) {
   %ret = trunc nuw i8 %lshr to i1
   ret i1 %ret
 }
+

>From 5e23c6b4a8138774b81dafbb36835cae4a334e2f Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 7 Sep 2026 16:42:39 +0100
Subject: [PATCH 2/2] [InstCombine] Use common flags for shl in shl/trunc fold.

As pointed out post-commit in
https://github.com/llvm/llvm-project/pull/219443, whether the flags from
the trunc can be transferred directly to the shl depends on the involved
types: https://alive2.llvm.org/ce/z/zN_TFq

For now, just use the common flags.
---
 llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp | 8 +++-----
 llvm/test/Transforms/InstCombine/trunc.ll            | 9 ++++-----
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index fb56bcc6bea26..54d811fa3e923 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1236,18 +1236,16 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
       APInt Threshold = APInt(C->getType()->getScalarSizeInBits(), DestWidth);
       if (match(C, m_SpecificInt_ICMP(ICmpInst::ICMP_ULT, Threshold))) {
         // If neither the wide shift nor the truncate wrap, propagate the wrap
-        // flags on the new truncate.
+        // flags on the new truncate and shift.
         auto *WideShl = cast<OverflowingBinaryOperator>(Src);
         bool NUW = Trunc.hasNoUnsignedWrap() && WideShl->hasNoUnsignedWrap();
         bool NSW = Trunc.hasNoSignedWrap() && WideShl->hasNoSignedWrap();
         Value *NewTrunc = Builder.CreateTrunc(A, DestTy, A->getName() + ".tr",
                                               /*IsNUW=*/NUW, /*IsNSW=*/NSW);
-        // The original flags from the truncate can be propagated directly to
-        // the shift.
         auto *NewShl = BinaryOperator::Create(
             Instruction::Shl, NewTrunc, ConstantExpr::getTrunc(C, DestTy));
-        NewShl->setHasNoUnsignedWrap(Trunc.hasNoUnsignedWrap());
-        NewShl->setHasNoSignedWrap(Trunc.hasNoSignedWrap());
+        NewShl->setHasNoUnsignedWrap(NUW);
+        NewShl->setHasNoSignedWrap(NSW);
         return NewShl;
       }
     }
diff --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index 868f3a7ebd666..9e41544043d51 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -664,7 +664,7 @@ define i32 @trunc_nuw_shl_nuw_nsw_3_i32_i64(i64 %val) {
 define i32 @trunc_nuw_nsw_shl_nsw_3_i32_i64(i64 %val) {
 ; CHECK-LABEL: @trunc_nuw_nsw_shl_nsw_3_i32_i64(
 ; CHECK-NEXT:    [[VAL_TR:%.*]] = trunc nsw i64 [[VAL:%.*]] to i32
-; CHECK-NEXT:    [[TRUNC:%.*]] = shl nuw nsw i32 [[VAL_TR]], 3
+; CHECK-NEXT:    [[TRUNC:%.*]] = shl nsw i32 [[VAL_TR]], 3
 ; CHECK-NEXT:    ret i32 [[TRUNC]]
 ;
   %shl = shl nsw i64 %val, 3
@@ -676,7 +676,7 @@ define i32 @trunc_nuw_nsw_shl_nsw_3_i32_i64(i64 %val) {
 define i32 @trunc_nuw_shl_3_i32_i64(i64 %val) {
 ; CHECK-LABEL: @trunc_nuw_shl_3_i32_i64(
 ; CHECK-NEXT:    [[VAL_TR:%.*]] = trunc i64 [[VAL:%.*]] to i32
-; CHECK-NEXT:    [[TRUNC:%.*]] = shl nuw i32 [[VAL_TR]], 3
+; CHECK-NEXT:    [[TRUNC:%.*]] = shl i32 [[VAL_TR]], 3
 ; CHECK-NEXT:    ret i32 [[TRUNC]]
 ;
   %shl = shl i64 %val, 3
@@ -700,7 +700,7 @@ define i32 @trunc_shl_nuw_nsw_3_i32_i64(i64 %val) {
 define i32 @trunc_nuw_shl_nsw_3_i32_i64(i64 %val) {
 ; CHECK-LABEL: @trunc_nuw_shl_nsw_3_i32_i64(
 ; CHECK-NEXT:    [[VAL_TR:%.*]] = trunc i64 [[VAL:%.*]] to i32
-; CHECK-NEXT:    [[TRUNC:%.*]] = shl nuw i32 [[VAL_TR]], 3
+; CHECK-NEXT:    [[TRUNC:%.*]] = shl i32 [[VAL_TR]], 3
 ; CHECK-NEXT:    ret i32 [[TRUNC]]
 ;
   %shl = shl nsw i64 %val, 3
@@ -711,7 +711,7 @@ define i32 @trunc_nuw_shl_nsw_3_i32_i64(i64 %val) {
 define i4 @trunc_nuw_shl_i6_to_i4(i6 %x, i6 %y) {
 ; CHECK-LABEL: @trunc_nuw_shl_i6_to_i4(
 ; CHECK-NEXT:    [[X_TR:%.*]] = trunc i6 [[X:%.*]] to i4
-; CHECK-NEXT:    [[TRUNC:%.*]] = shl nuw i4 [[X_TR]], 3
+; CHECK-NEXT:    [[TRUNC:%.*]] = shl i4 [[X_TR]], 3
 ; CHECK-NEXT:    ret i4 [[TRUNC]]
 ;
   %shl = shl i6 %x, 3
@@ -1564,4 +1564,3 @@ define i1 @neg_trunc_nuw_lshr(i8 %x, i8 %c) {
   %ret = trunc nuw i8 %lshr to i1
   ret i1 %ret
 }
-