diff --git a/Test/Passes/InstructionSelection/RISCV64/binop_constant_decoding.mlir b/Test/Passes/InstructionSelection/RISCV64/binop_constant_decoding.mlir
new file mode 100644
index 0000000000..e2f0a75780
--- /dev/null
+++ b/Test/Passes/InstructionSelection/RISCV64/binop_constant_decoding.mlir
@@ -0,0 +1,21 @@
+// RUN: veir-opt %s -p=isel-sdag-riscv64 | filecheck %s
+
+"builtin.module"() ({
+  "func.func"() <{sym_name = "binop_constants", function_type = (i64, i32) -> (i64, i64, i64, i32)}> ({
+  ^bb0(%x: i64, %y: i32):
+    %minusOne = "llvm.mlir.constant"() <{value = 255 : i8}> : () -> i64
+    %one = "llvm.mlir.constant"() <{value = -1 : i1}> : () -> i64
+    %sum = "llvm.add"(%x, %minusOne) : (i64, i64) -> i64
+    %masked = "llvm.and"(%x, %minusOne) : (i64, i64) -> i64
+    %shifted = "llvm.shl"(%x, %one) : (i64, i64) -> i64
+    %truncatedOne = "llvm.mlir.constant"() <{value = 4294967297 : i64}> : () -> i32
+    %sum32 = "llvm.add"(%y, %truncatedOne) : (i32, i32) -> i32
+    "func.return"(%sum, %masked, %shifted, %sum32) : (i64, i64, i64, i32) -> ()
+  }) : () -> ()
+}) : () -> ()
+
+// CHECK-LABEL: func.func @binop_constants
+// CHECK: "riscv.addi"({{.*}}) <{"value" = -1 : i64}>
+// CHECK: "riscv.andi"({{.*}}) <{"value" = -1 : i64}>
+// CHECK: "riscv.slli"({{.*}}) <{"value" = 1 : i64}>
+// CHECK: "riscv.addiw"({{.*}}) <{"value" = 1 : i64}>
diff --git a/Test/Passes/RISCVCombines/mul_neg_one_flags.mlir b/Test/Passes/RISCVCombines/mul_neg_one_flags.mlir
new file mode 100644
index 0000000000..4535ed13c6
--- /dev/null
+++ b/Test/Passes/RISCVCombines/mul_neg_one_flags.mlir
@@ -0,0 +1,29 @@
+// RUN: veir-opt %s -p=riscv-combine | filecheck %s
+
+// 1 * 255 is defined unsigned, but 0 - 1 underflows. Multiplication by all-ones
+// can become negation only if nuw is dropped. Signed negation preserves nsw.
+// overflowFlags: 1 = nsw (no signed wrap), 2 = nuw (no unsigned wrap), 3 = both.
+"builtin.module"() ({
+  "func.func"() <{sym_name = "mul_neg_one", function_type = (i8) -> (i8, i8, i8, i8)}> ({
+  ^bb0(%x: i8):
+    %unsigned = "llvm.mlir.constant"() <{value = 255 : i8}> : () -> i8
+    %signed = "llvm.mlir.constant"() <{value = -1 : i8}> : () -> i8
+    %a = "llvm.mul"(%x, %unsigned) <{overflowFlags = 2 : i32}> : (i8, i8) -> i8
+    %b = "llvm.mul"(%x, %signed) <{overflowFlags = 2 : i32}> : (i8, i8) -> i8
+    %c = "llvm.mul"(%x, %signed) <{overflowFlags = 1 : i32}> : (i8, i8) -> i8
+    %d = "llvm.mul"(%x, %signed) <{overflowFlags = 3 : i32}> : (i8, i8) -> i8
+    "func.return"(%a, %b, %c, %d) : (i8, i8, i8, i8) -> ()
+  }) : () -> ()
+}) : () -> ()
+
+// CHECK-LABEL: func.func @mul_neg_one
+// CHECK-SAME: (%[[X:.*]]: i8)
+// CHECK-NEXT: %[[Z0:.*]] = "llvm.mlir.constant"() <{"value" = 0 : i8}>
+// CHECK-NEXT: %[[A:.*]] = "llvm.sub"(%[[Z0]], %[[X]]) : (i8, i8) -> i8
+// CHECK-NEXT: %[[Z1:.*]] = "llvm.mlir.constant"() <{"value" = 0 : i8}>
+// CHECK-NEXT: %[[B:.*]] = "llvm.sub"(%[[Z1]], %[[X]]) : (i8, i8) -> i8
+// CHECK-NEXT: %[[Z2:.*]] = "llvm.mlir.constant"() <{"value" = 0 : i8}>
+// CHECK-NEXT: %[[C:.*]] = "llvm.sub"(%[[Z2]], %[[X]]) <{"overflowFlags" = 1 : i32}> : (i8, i8) -> i8
+// CHECK-NEXT: %[[Z3:.*]] = "llvm.mlir.constant"() <{"value" = 0 : i8}>
+// CHECK-NEXT: %[[D:.*]] = "llvm.sub"(%[[Z3]], %[[X]]) <{"overflowFlags" = 1 : i32}> : (i8, i8) -> i8
+// CHECK-NEXT: "func.return"(%[[A]], %[[B]], %[[C]], %[[D]]) : (i8, i8, i8, i8) -> ()
diff --git a/Test/Passes/RISCVCombines/select_i1.mlir b/Test/Passes/RISCVCombines/select_i1.mlir
new file mode 100644
index 0000000000..30c5f2a260
--- /dev/null
+++ b/Test/Passes/RISCVCombines/select_i1.mlir
@@ -0,0 +1,20 @@
+// RUN: veir-opt %s -p=riscv-combine | filecheck %s
+
+// A boolean result needs the condition or its inverse, without an extension.
+"builtin.module"() ({
+  "func.func"() <{sym_name = "select_i1", function_type = (i1) -> (i1, i1)}> ({
+  ^bb0(%cond: i1):
+    %one = "llvm.mlir.constant"() <{value = 1 : i1}> : () -> i1
+    %minusOne = "llvm.mlir.constant"() <{value = -1 : i1}> : () -> i1
+    %zero = "llvm.mlir.constant"() <{value = 0 : i1}> : () -> i1
+    %a = "llvm.select"(%cond, %one, %zero) : (i1, i1, i1) -> i1
+    %b = "llvm.select"(%cond, %zero, %minusOne) : (i1, i1, i1) -> i1
+    "func.return"(%a, %b) : (i1, i1) -> ()
+  }) : () -> ()
+}) : () -> ()
+
+// CHECK-LABEL: func.func @select_i1
+// CHECK-SAME: (%[[COND:.*]]: i1)
+// CHECK-NEXT: %[[ONE:.*]] = "llvm.mlir.constant"() <{"value" = -1 : i1}> : () -> i1
+// CHECK-NEXT: %[[NOT:.*]] = "llvm.xor"(%[[COND]], %[[ONE]]) : (i1, i1) -> i1
+// CHECK-NEXT: "func.return"(%[[COND]], %[[NOT]]) : (i1, i1) -> ()
diff --git a/Veir/Passes/InstCombine.lean b/Veir/Passes/InstCombine.lean
index 0603c9f630..3e911ab53f 100644
--- a/Veir/Passes/InstCombine.lean
+++ b/Veir/Passes/InstCombine.lean
@@ -57,9 +57,7 @@ def mulIOneToX_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
     Option (WfIRContext OpCode × Option (Array OperationPtr × Array ValuePtr)) := do
   let some (lhs, rhs, _) := matchMuli op ctx.raw
     | return (ctx, none)
-  let some cst := matchConstantIntVal rhs ctx.raw
-    | return (ctx, none)
-  if cst ≠ 1 then
+  if !isConstantOne rhs ctx.raw then
     return (ctx, none)
   some (ctx, some (#[], #[lhs]))
 
diff --git a/Veir/Passes/Matching/LLVM/Basic.lean b/Veir/Passes/Matching/LLVM/Basic.lean
index d5da98b40a..f52b890dea 100644
--- a/Veir/Passes/Matching/LLVM/Basic.lean
+++ b/Veir/Passes/Matching/LLVM/Basic.lean
@@ -51,6 +51,7 @@ def matchXori (op : OperationPtr) (ctx : IRContext OpCode) :
   let (op, _) ← matchOp op ctx (Llvm.xor) 2
   return (op[0]!, op[1]!)
 
+/-- Match the raw integer attribute; use `matchConstantIntVal` for the result's value. -/
 def matchConstantIntOp (op : OperationPtr) (ctx : IRContext OpCode) :
     Option IntegerAttr := do
   let Llvm.mlir__constant := toDialect? Llvm (op.getOpType! ctx) | none
@@ -58,14 +59,20 @@ def matchConstantIntOp (op : OperationPtr) (ctx : IRContext OpCode) :
   let .integer intAttr := properties.value | none
   return intAttr
 
-/-- Match the raw integer attribute value of an LLVM constant, without adjusting
-it to the attribute or result width. -/
 def matchConstantIntVal (val : ValuePtr) (ctx : IRContext OpCode) :
     Option Int := do
   let .opResult opResultPtr := val | none
   let op := opResultPtr.op
   let attr ← matchConstantIntOp op ctx
-  return attr.value
+  let .integerType type := (val.getType! ctx).val | none
+  return (BitVec.ofInt type.bitwidth (decodeLLVMIntegerConstant attr)).toInt
+
+/-- Recognize the one bit pattern, including i1 true whose signed value is -1. -/
+def isConstantOne (val : ValuePtr) (ctx : IRContext OpCode) : Bool :=
+  match matchConstantIntVal val ctx, (val.getType! ctx).val with
+  | some value, .integerType type =>
+    (BitVec.ofInt type.bitwidth value).toNat == 1
+  | _, _ => false
 
 /-- Match a constant integer with value zero, returning `val` itself. -/
 def matchConstantZero (val : ValuePtr) (ctx : IRContext OpCode) : Option ValuePtr := do
diff --git a/Veir/Passes/Matching/LLVM/Lemmas.lean b/Veir/Passes/Matching/LLVM/Lemmas.lean
index 0a59020499..7818512d21 100644
--- a/Veir/Passes/Matching/LLVM/Lemmas.lean
+++ b/Veir/Passes/Matching/LLVM/Lemmas.lean
@@ -110,9 +110,10 @@ theorem matchConstantIntOp_implies {op : OperationPtr} {ctx : IRContext OpCode}
 /-- What matching a constant integer value (via `matchConstantIntVal`) syntactically guarantees. -/
 theorem matchConstantIntVal_implies {val : ValuePtr} {ctx : IRContext OpCode} {value} :
     matchConstantIntVal val ctx = some value →
-    ∃ opResultPtr intAttr, val = .opResult opResultPtr ∧
-      matchConstantIntOp opResultPtr.op ctx = some intAttr ∧
-      intAttr.value = value := by
+    ∃ opResultPtr attr type, val = .opResult opResultPtr ∧
+      matchConstantIntOp opResultPtr.op ctx = some attr ∧
+      (val.getType! ctx).val = .integerType type ∧
+      value = (BitVec.ofInt type.bitwidth (decodeLLVMIntegerConstant attr)).toInt := by
   intro hmatch
   simp only [matchConstantIntVal, bind, Option.bind, pure] at hmatch
   grind
diff --git a/Veir/Passes/RISCVCombines/Combine.lean b/Veir/Passes/RISCVCombines/Combine.lean
index 09b9076f9e..88b541b692 100644
--- a/Veir/Passes/RISCVCombines/Combine.lean
+++ b/Veir/Passes/RISCVCombines/Combine.lean
@@ -63,8 +63,7 @@ def select_same_val_self (rewriter : PatternRewriter OpCode) (op : OperationPtr)
 def select_constant_cmp_true_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
     Option (WfIRContext OpCode × Option (Array OperationPtr × Array ValuePtr)) := do
   let some (cond, tval, _fval) := matchSelect op ctx.raw | return (ctx, none)
-  let some cst := matchConstantIntVal cond ctx.raw | return (ctx, none)
-  if cst ≠ 1 then return (ctx, none)
+  if !isConstantOne cond ctx.raw then return (ctx, none)
   some (ctx, some (#[], #[tval]))
 
 def select_constant_cmp_true (rewriter : PatternRewriter OpCode) (op : OperationPtr)
@@ -958,6 +957,9 @@ def select_neg1_0_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
   if ct ≠ -1 then return (ctx, none)
   let some cf := matchConstantIntVal fv ctx.raw | return (ctx, none)
   if cf ≠ 0 then return (ctx, none)
+  -- At i1, -1 is true and no extension is needed.
+  if (op.getResult 0 : ValuePtr).getType! ctx.raw = IntegerType.mk 1 then
+    return (ctx, some (#[], #[cond]))
   let (ctx, newOp) ← WfRewriter.createOp! ctx Llvm.sext #[(op.getResult 0 : ValuePtr).getType! ctx.raw] #[cond]
     #[] #[] () none
   some (ctx, some (#[newOp], #[newOp.getResult 0]))
@@ -1002,6 +1004,8 @@ def select_0_neg1_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
     #[] #[] m1 none
   let (ctx, ncond) ← WfRewriter.createOp! ctx Llvm.xor #[cond.getType! ctx.raw] #[cond, (c1.getResult 0)]
     #[] #[] () none
+  if (op.getResult 0 : ValuePtr).getType! ctx.raw = IntegerType.mk 1 then
+    return (ctx, some (#[c1, ncond], #[ncond.getResult 0]))
   let (ctx, newOp) ← WfRewriter.createOp! ctx Llvm.sext #[(op.getResult 0 : ValuePtr).getType! ctx.raw] #[(ncond.getResult 0)]
     #[] #[] () none
   some (ctx, some (#[c1, ncond, newOp], #[newOp.getResult 0]))
diff --git a/Veir/Passes/RISCVCombines/MIRCombinesVeir.lean b/Veir/Passes/RISCVCombines/MIRCombinesVeir.lean
index f566a33a2b..5f622678b9 100644
--- a/Veir/Passes/RISCVCombines/MIRCombinesVeir.lean
+++ b/Veir/Passes/RISCVCombines/MIRCombinesVeir.lean
@@ -107,8 +107,7 @@ def right_identity_zero_6 (rewriter : PatternRewriter OpCode) (op : OperationPtr
 def right_identity_one_int_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
     Option (WfIRContext OpCode × Option (Array OperationPtr × Array ValuePtr)) := do
   let some (x, rhs, _props) := matchMul op ctx.raw | return (ctx, none)
-  let some cst := matchConstantIntVal rhs ctx.raw | return (ctx, none)
-  if cst ≠ 1 then return (ctx, none)
+  if !isConstantOne rhs ctx.raw then return (ctx, none)
   some (ctx, some (#[], #[x]))
 
 def right_identity_one_int (rewriter : PatternRewriter OpCode) (op : OperationPtr)
@@ -182,15 +181,16 @@ def binop_right_to_zero (rewriter : PatternRewriter OpCode) (op : OperationPtr)
 
 def mul_by_neg_one_local (ctx : WfIRContext OpCode) (op : OperationPtr) :
     Option (WfIRContext OpCode × Option (Array OperationPtr × Array ValuePtr)) := do
-  let some (x, rhs, _props) := matchMul op ctx.raw | return (ctx, none)
+  let some (x, rhs, props) := matchMul op ctx.raw | return (ctx, none)
   let some cst := matchConstantIntVal rhs ctx.raw | return (ctx, none)
   if cst ≠ -1 then return (ctx, none)
   let .integerType ctype := (x.getType! ctx.raw).val | return (ctx, none)
   let cstOpProp := LLVMConstantProperties.mk (.integer (IntegerAttr.mk (0) ctype))
   let (ctx, cstOp) ← WfRewriter.createOp! ctx Llvm.mlir__constant #[x.getType! ctx.raw] #[]
     #[] #[] cstOpProp none
+  -- Multiplying 1 by all-ones does not overflow unsigned, but 0 - 1 does.
   let (ctx, newOp) ← WfRewriter.createOp! ctx Llvm.sub #[x.getType! ctx.raw] #[(cstOp.getResult 0), x]
-    #[] #[] _props none
+    #[] #[] { props with nuw := false } none
   some (ctx, some (#[cstOp, newOp], #[newOp.getResult 0]))
 
 def mul_by_neg_one (rewriter : PatternRewriter OpCode) (op : OperationPtr)