[HLSL] Add abs library function

[beanz]

Summary

This change exposes the abs library function for HLSL scalar types. Abs
is supported for all scalar, vector and matrix types. This patch only
adds a subset of scalar type support.

Fixes #57100 (https://llvm.org/pr57100)

The full documentation of the HLSL abs function is available here:
https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-abs

[python3kgae]

clang/lib/Headers/hlsl/hlsl_intrinsics.h

17

How to add 16bit abs?
Something like this?

#ifdef HLSL_ENABLE_16_BIT
attribute((clang_builtin_alias(builtin_fabsf16))) half abs(half In);
#endif

[beanz]

Adding missing test coverage for 64-bit types

[beanz]

Updating with a fix for half type parameters. This addresses the issue I filed here: https://github.com/llvm/llvm-project/issues/57100

The issue was caused by HLSL not implying options to allow half types to be used as parameters and return types.

[bob80905]

clang/test/CodeGenHLSL/builtins/abs.hlsl

21

Is there a reason why we don't check the function parameter here, but we do check for function parameters when actually calling the builtin intrinsic on the next call line?

[bob80905]

clang/include/clang/Driver/Options.td

6130 ↗

(On Diff #454962)

In Clang.cpp, you removed CmdArgs.push_back("-fallow-half-arguments-and-returns");
But here, you add an extra condition to imply fnative-half-arguments-and-returns
Wouldn't this change the behavior? or is it really alright for the definition below, fallow_half_arguments_and_returns , to be missing hlsl.KeyPath in the implication list?

[beanz]

clang/include/clang/Driver/Options.td

6130 ↗

(On Diff #454962)

The big issue with the change in Clang.cpp is that it only works if you invoke clang-dxc, not if you invoke clang directly with an hlsl source. That has the undesirable effect of making clang HLSL support only work through the clang-dxc driver, which isn't what we want.

There is also a subtle difference between native and non-native half arguments and returns, where non-native half arguments and returns are actually passed as larger float types. Which also isn't correct for HLSL.

This change follows how OpenCL works, and enables -fnative-half-arguments-and-returns whenever the HLSL language mode is enabled, and that option also implies -fhalf-arguments-and-returns.

The big functional change here is that you don't need to ever manually pass these flags to clang if your language is HLSL, so you can use any clang driver entry and get the correct behavior for the language.

clang/test/CodeGenHLSL/builtins/abs.hlsl

21

The check lines on the function declarations are really just to make sure that the next match occurs in the function body. The real test is to make sure the body gets generated correctly.

That said, matching the full mangled name has the same impact as matching the parameters too because the name mangling encodes the parameters too. The NN specifies the return and first argument parameters are doubles.

[bogner]

LGTM. Make sure you update the description from saying that this doesn't handle half because of https://llvm.org/pr57100 to saying that it fixes the bug instead.

clang/lib/Headers/hlsl/hlsl_intrinsics.h

 //===----- hlsl_intrinsics.h - HLSL definitions for intrinsics ----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _HLSL_HLSL_INTRINSICS_H_
 #define _HLSL_HLSL_INTRINSICS_H_
 
 __attribute__((clang_builtin_alias(__builtin_hlsl_wave_active_count_bits))) uint
 WaveActiveCountBits(bool bBit);
 
+__attribute__((clang_builtin_alias(__builtin_abs))) int abs(int In);
+__attribute__((clang_builtin_alias(__builtin_labs))) int64_t abs(int64_t In);
+__attribute__((clang_builtin_alias(__builtin_fabsf))) float abs(float In);
+__attribute__((clang_builtin_alias(__builtin_fabs))) double abs(double In);
+
 #endif //_HLSL_HLSL_INTRINSICS_H_

clang/test/CodeGenHLSL/builtins/abs.hlsl

+// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -triple dxil-pc-shadermodel6.3-library %s -emit-llvm -disable-llvm-passes -o - | FileCheck %s
+
+
+float abs_float(float X) {
+  return abs(X);
+}
+
+// CHECK: define noundef float @"?abs_float@@YAMM@Z"(float
+// CHECK: call float @llvm.fabs.f32(float %0)
+
+int abs_int(int X) {
+  return abs(X);
+}
+
+// CHECK: define noundef i32 @"?abs_int@@YAHH@Z"(i32
+// CHECK:         [[A_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    store i32 [[A:%.*]], ptr [[A_ADDR]], align 4
+// CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr [[A_ADDR]], align 4
+// CHECK-NEXT:    [[NEG:%.*]] = sub nsw i32 0, [[TMP0]]
+// CHECK-NEXT:    [[ABSCOND:%.*]] = icmp slt i32 [[TMP0]], 0
+// CHECK-NEXT:    [[ABS:%.*]] = select i1 [[ABSCOND]], i32 [[NEG]], i32 [[TMP0]]
+// CHECK-NEXT:    ret i32 [[ABS]]