[InstCombine] Fold (icmp pred (trunc nuw/nsw X), C) -> (icmp pred X, (zext/sext C))

This is valid as long as the sign of the wrap flag doesn't differ from
the sign of the `pred`.

Proofs: https://alive2.llvm.org/ce/z/35NsrR

NB: The online Alive2 hasn't been updated with `trunc nuw/nsw`
support, so the proofs must be reproduced locally.

Closes #87935

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -1409,6 +1409,19 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
                                                      const APInt &C) {
   ICmpInst::Predicate Pred = Cmp.getPredicate();
   Value *X = Trunc->getOperand(0);
+  Type *SrcTy = X->getType();
+  unsigned DstBits = Trunc->getType()->getScalarSizeInBits(),
+           SrcBits = SrcTy->getScalarSizeInBits();
+
+  // Match (icmp pred (trunc nuw/nsw X), C)
+  // Which we can convert to (icmp pred X, (sext/zext C))
+  if (shouldChangeType(Trunc->getType(), SrcTy)) {
+    if (Trunc->hasNoSignedWrap())
+      return new ICmpInst(Pred, X, ConstantInt::get(SrcTy, C.sext(SrcBits)));
+    if (!Cmp.isSigned() && Trunc->hasNoUnsignedWrap())
+      return new ICmpInst(Pred, X, ConstantInt::get(SrcTy, C.zext(SrcBits)));
+  }
+
   if (C.isOne() && C.getBitWidth() > 1) {
     // icmp slt trunc(signum(V)) 1 --> icmp slt V, 1
     Value *V = nullptr;
@@ -1417,10 +1430,6 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
                           ConstantInt::get(V->getType(), 1));
   }
 
-  Type *SrcTy = X->getType();
-  unsigned DstBits = Trunc->getType()->getScalarSizeInBits(),
-           SrcBits = SrcTy->getScalarSizeInBits();
-
   // TODO: Handle any shifted constant by subtracting trailing zeros.
   // TODO: Handle non-equality predicates.
   Value *Y;

llvm/test/Transforms/InstCombine/icmp-trunc.ll

@@ -558,8 +558,7 @@ define i1 @shl1_trunc_sgt4(i32 %a) {
 
 define i1 @eq_nuw(i32 %x) {
 ; DL64-LABEL: @eq_nuw(
-; DL64-NEXT:    [[TMP1:%.*]] = and i32 [[X:%.*]], 255
-; DL64-NEXT:    [[R:%.*]] = icmp eq i32 [[TMP1]], 123
+; DL64-NEXT:    [[R:%.*]] = icmp eq i32 [[X:%.*]], 123
 ; DL64-NEXT:    ret i1 [[R]]
 ;
 ; DL8-LABEL: @eq_nuw(
@@ -573,22 +572,32 @@ define i1 @eq_nuw(i32 %x) {
 }
 
 define i1 @ult_nuw(i32 %x) {
-; CHECK-LABEL: @ult_nuw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T]], 45
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @ult_nuw(
+; DL64-NEXT:    [[R:%.*]] = icmp ult i32 [[X:%.*]], 45
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @ult_nuw(
+; DL8-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp ult i8 [[T]], 45
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nuw i32 %x to i8
   %r = icmp ult i8 %t, 45
   ret i1 %r
 }
 
 define i1 @ule_nuw(i32 %x) {
-; CHECK-LABEL: @ule_nuw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[T]], 46
-; CHECK-NEXT:    call void @use(i8 [[T]])
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @ule_nuw(
+; DL64-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL64-NEXT:    [[R:%.*]] = icmp ult i32 [[X]], 46
+; DL64-NEXT:    call void @use(i8 [[T]])
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @ule_nuw(
+; DL8-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp ult i8 [[T]], 46
+; DL8-NEXT:    call void @use(i8 [[T]])
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nuw i32 %x to i8
   %r = icmp ule i8 %t, 45
@@ -597,22 +606,32 @@ define i1 @ule_nuw(i32 %x) {
 }
 
 define i1 @ugt_nuw(i32 %x) {
-; CHECK-LABEL: @ugt_nuw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[T]], 12
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @ugt_nuw(
+; DL64-NEXT:    [[R:%.*]] = icmp ugt i32 [[X:%.*]], 12
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @ugt_nuw(
+; DL8-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp ugt i8 [[T]], 12
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nuw i32 %x to i8
   %r = icmp ugt i8 %t, 12
   ret i1 %r
 }
 
 define i1 @uge_nuw(i32 %x) {
-; CHECK-LABEL: @uge_nuw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt i8 [[T]], 98
-; CHECK-NEXT:    call void @use(i8 [[T]])
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @uge_nuw(
+; DL64-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL64-NEXT:    [[R:%.*]] = icmp ugt i32 [[X]], 98
+; DL64-NEXT:    call void @use(i8 [[T]])
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @uge_nuw(
+; DL8-NEXT:    [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp ugt i8 [[T]], 98
+; DL8-NEXT:    call void @use(i8 [[T]])
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nuw i32 %x to i8
   %r = icmp uge i8 %t, 99
@@ -646,8 +665,7 @@ define i1 @sgt_nuw_fail(i32 %x) {
 
 define i1 @ne_nsw(i32 %x) {
 ; DL64-LABEL: @ne_nsw(
-; DL64-NEXT:    [[TMP1:%.*]] = and i32 [[X:%.*]], 255
-; DL64-NEXT:    [[R:%.*]] = icmp ne i32 [[TMP1]], 133
+; DL64-NEXT:    [[R:%.*]] = icmp ne i32 [[X:%.*]], -123
 ; DL64-NEXT:    ret i1 [[R]]
 ;
 ; DL8-LABEL: @ne_nsw(
@@ -661,22 +679,32 @@ define i1 @ne_nsw(i32 %x) {
 }
 
 define i1 @slt_nsw(i32 %x) {
-; CHECK-LABEL: @slt_nsw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nsw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp slt i8 [[T]], 45
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @slt_nsw(
+; DL64-NEXT:    [[R:%.*]] = icmp slt i32 [[X:%.*]], 45
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @slt_nsw(
+; DL8-NEXT:    [[T:%.*]] = trunc nsw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp slt i8 [[T]], 45
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nsw i32 %x to i8
   %r = icmp slt i8 %t, 45
   ret i1 %r
 }
 
 define i1 @sle_nsw(i16 %x) {
-; CHECK-LABEL: @sle_nsw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nsw i16 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp slt i8 [[T]], 46
-; CHECK-NEXT:    call void @use(i8 [[T]])
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @sle_nsw(
+; DL64-NEXT:    [[T:%.*]] = trunc nsw i16 [[X:%.*]] to i8
+; DL64-NEXT:    [[R:%.*]] = icmp slt i16 [[X]], 46
+; DL64-NEXT:    call void @use(i8 [[T]])
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @sle_nsw(
+; DL8-NEXT:    [[T:%.*]] = trunc nsw i16 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp slt i8 [[T]], 46
+; DL8-NEXT:    call void @use(i8 [[T]])
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nsw i16 %x to i8
   %r = icmp sle i8 %t, 45
@@ -685,22 +713,32 @@ define i1 @sle_nsw(i16 %x) {
 }
 
 define i1 @sgt_nsw(i32 %x) {
-; CHECK-LABEL: @sgt_nsw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nsw i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i8 [[T]], -12
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @sgt_nsw(
+; DL64-NEXT:    [[R:%.*]] = icmp sgt i32 [[X:%.*]], -12
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @sgt_nsw(
+; DL8-NEXT:    [[T:%.*]] = trunc nsw i32 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp sgt i8 [[T]], -12
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nsw i32 %x to i8
   %r = icmp sgt i8 %t, -12
   ret i1 %r
 }
 
 define i1 @sge_nsw(i64 %x) {
-; CHECK-LABEL: @sge_nsw(
-; CHECK-NEXT:    [[T:%.*]] = trunc nsw i64 [[X:%.*]] to i8
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i8 [[T]], 98
-; CHECK-NEXT:    call void @use(i8 [[T]])
-; CHECK-NEXT:    ret i1 [[R]]
+; DL64-LABEL: @sge_nsw(
+; DL64-NEXT:    [[T:%.*]] = trunc nsw i64 [[X:%.*]] to i8
+; DL64-NEXT:    [[R:%.*]] = icmp sgt i64 [[X]], 98
+; DL64-NEXT:    call void @use(i8 [[T]])
+; DL64-NEXT:    ret i1 [[R]]
+;
+; DL8-LABEL: @sge_nsw(
+; DL8-NEXT:    [[T:%.*]] = trunc nsw i64 [[X:%.*]] to i8
+; DL8-NEXT:    [[R:%.*]] = icmp sgt i8 [[T]], 98
+; DL8-NEXT:    call void @use(i8 [[T]])
+; DL8-NEXT:    ret i1 [[R]]
 ;
   %t = trunc nsw i64 %x to i8
   %r = icmp sge i8 %t, 99