[mlir][arith] Match folding of arith.remf to llvm.frem semantics (llvm#96537)

There are multiple ways to define a remainder operation. Depending on
the definition, the result could be either always positive or have the
sign of the dividend.
The pattern lowering `arith.remf` to LLVM assumes that the semantics
match `llvm.frem`, which seems to be reasonable. The folder, however, is
implemented via `APFloat::remainder()` which has different semantics.

This patch matches the folding behaviour to lowering behavior by using
`APFloat::mod()`, which matches the behavior of `llvm.frem` and libm's
`fmod()`. It also updates the documentation of `arith.remf` to explain
this behavior: The sign of the result of the remainder operation always
matches the sign of the dividend (LHS operand).

frem documentation: https://llvm.org/docs/LangRef.html#frem-instruction

Fix llvm#94431

---------

Co-authored-by: Jakub Kuderski <[email protected]>

Author: 2 people

mlir/include/mlir/Dialect/Arith/IR/ArithOps.td

@@ -1133,6 +1133,10 @@ def Arith_DivFOp : Arith_FloatBinaryOp<"divf"> {
 
 def Arith_RemFOp : Arith_FloatBinaryOp<"remf"> {
   let summary = "floating point division remainder operation";
+  let description = [{
+    Returns the floating point division remainder.
+    The remainder has the same sign as the dividend (lhs operand).
+  }];
   let hasFolder = 1;
 }
 

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1204,7 +1204,10 @@ OpFoldResult arith::RemFOp::fold(FoldAdaptor adaptor) {
   return constFoldBinaryOp<FloatAttr>(adaptor.getOperands(),
                                       [](const APFloat &a, const APFloat &b) {
                                         APFloat result(a);
-                                        (void)result.remainder(b);
+                                        // APFloat::mod() offers the remainder
+                                        // behavior we want, i.e. the result has
+                                        // the sign of LHS operand.
+                                        (void)result.mod(b);
                                         return result;
                                       });
 }

mlir/test/Dialect/Arith/canonicalize.mlir

@@ -2467,7 +2467,7 @@ func.func @test_remsi_1(%arg : vector<4xi32>) -> (vector<4xi32>) {
 // -----
 
 // CHECK-LABEL: @test_remf(
-// CHECK: %[[res:.+]] = arith.constant -1.000000e+00 : f32
+// CHECK: %[[res:.+]] = arith.constant 1.000000e+00 : f32
 // CHECK: return %[[res]]
 func.func @test_remf() -> (f32) {
   %v1 = arith.constant 3.0 : f32
@@ -2476,11 +2476,24 @@ func.func @test_remf() -> (f32) {
   return %0 : f32
 }
 
+// CHECK-LABEL: @test_remf2(
+// CHECK: %[[respos:.+]] = arith.constant 1.000000e+00 : f32
+// CHECK: %[[resneg:.+]] = arith.constant -1.000000e+00 : f32
+// CHECK: return %[[respos]], %[[resneg]]
+func.func @test_remf2() -> (f32, f32) {
+  %v1 = arith.constant 3.0 : f32
+  %v2 = arith.constant -2.0 : f32
+  %v3 = arith.constant -3.0 : f32
+  %0 = arith.remf %v1, %v2 : f32
+  %1 = arith.remf %v3, %v2 : f32
+  return %0, %1 : f32, f32
+}
+
 // CHECK-LABEL: @test_remf_vec(
 // CHECK: %[[res:.+]] = arith.constant dense<[1.000000e+00, 0.000000e+00, -1.000000e+00, 0.000000e+00]> : vector<4xf32>
 // CHECK: return %[[res]]
 func.func @test_remf_vec() -> (vector<4xf32>) {
-  %v1 = arith.constant dense<[1.0, 2.0, 3.0, 4.0]> : vector<4xf32>
+  %v1 = arith.constant dense<[1.0, 2.0, -3.0, 4.0]> : vector<4xf32>
   %v2 = arith.constant dense<[2.0, 2.0, 2.0, 2.0]> : vector<4xf32>
   %0 = arith.remf %v1, %v2 : vector<4xf32>
   return %0 : vector<4xf32>