[compiler-rt][ARM] Optimized mulsf3 and divsf3 (#161546)

This commit adds optimized assembly versions of single-precision float
multiplication and division. Both functions are implemented in a style
that can be assembled as either of Arm and Thumb2; for multiplication, a
separate implementation is provided for Thumb1. Also, extensive new
tests are added for multiplication and division.

These implementations can be removed from the build by defining the
cmake variable COMPILER_RT_ARM_OPTIMIZED_FP=OFF.

Outlying parts of the functionality which are not on the fast path, such
as NaN handling and underflow, are handled in helper functions written
in C. These can be shared between the Arm/Thumb2 and Thumb1
implementations, and also reused by other optimized assembly functions
we hope to add in future.

Author: statham-arm

compiler-rt/cmake/Modules/CheckAssemblerFlag.cmake

@@ -0,0 +1,38 @@
+# Helper function to find out whether the assembler supports a particular
+# command-line flag. You'd like to use the standard check_compiler_flag(), but
+# that only supports a fixed list of languages, and ASM isn't one of them. So
+# we do it ourselves, by trying to assemble an empty source file.
+
+function(check_assembler_flag outvar flag)
+  if(NOT DEFINED "${outvar}")
+    if(NOT CMAKE_REQUIRED_QUIET)
+      message(CHECK_START "Checking for assembler flag ${flag}")
+    endif()
+
+    # Stop try_compile from attempting to link the result of the assembly, so
+    # that we don't depend on having a working linker, and also don't have to
+    # figure out what special symbol like _start needs to be defined in the
+    # test input.
+    #
+    # This change is made within the dynamic scope of this function, so
+    # CMAKE_TRY_COMPILE_TARGET_TYPE will be restored to its previous value on
+    # return.
+    set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+
+    # Try to assemble an empty file with a .S name, using the provided flag.
+    try_compile(success
+      SOURCE_FROM_CONTENT "CheckAssemblerFlag.s" ""
+      COMPILE_DEFINITIONS ${flag}
+      NO_CACHE)
+
+    if(NOT CMAKE_REQUIRED_QUIET)
+      if(success)
+        message(CHECK_PASS "Accepted")
+        set(${outvar} 1 CACHE INTERNAL "Test assembler flag ${flag}")
+      else()
+        message(CHECK_FAIL "Not accepted")
+        set(${outvar} "" CACHE INTERNAL "Test assembler flag ${flag}")
+      endif()
+    endif()
+  endif()
+endfunction()

compiler-rt/lib/builtins/CMakeLists.txt

@@ -60,6 +60,7 @@ endif()
 include(builtin-config-ix)
 include(CMakeDependentOption)
 include(CMakePushCheckState)
+include(CheckAssemblerFlag)
 
 option(COMPILER_RT_BUILTINS_HIDE_SYMBOLS
   "Do not export any symbols from the static library." ON)
@@ -423,6 +424,40 @@ set(arm_or_thumb2_base_SOURCES
   ${GENERIC_SOURCES}
 )
 
+option(COMPILER_RT_ARM_OPTIMIZED_FP
+  "On 32-bit Arm, use optimized assembly implementations of FP arithmetic. Likely to increase code size, but be faster." ON)
+
+if(COMPILER_RT_ARM_OPTIMIZED_FP AND BUILTIN_SUPPORTED_ARCH MATCHES "arm")
+  check_assembler_flag(COMPILER_RT_HAS_MIMPLICIT_IT -mimplicit-it=always)
+  if(COMPILER_RT_HAS_MIMPLICIT_IT)
+    set(implicit_it_flag -mimplicit-it=always)
+  else()
+    check_assembler_flag(
+      COMPILER_RT_HAS_WA_MIMPLICIT_IT -Wa,-mimplicit-it=always)
+    if(COMPILER_RT_HAS_WA_MIMPLICIT_IT)
+      set(implicit_it_flag -Wa,-mimplicit-it=always)
+    else()
+      message(WARNING "Don't know how to set the -mimplicit-it=always flag in this assembler; not including Arm optimized implementations")
+      set(implicit_it_flag "")
+    endif()
+  endif()
+
+  if(implicit_it_flag)
+    set(assembly_files
+      arm/mulsf3.S
+      arm/divsf3.S)
+    set_source_files_properties(${assembly_files}
+      PROPERTIES COMPILE_OPTIONS ${implicit_it_flag})
+    set(arm_or_thumb2_base_SOURCES
+      ${assembly_files}
+      arm/fnan2.c
+      arm/fnorm2.c
+      arm/funder.c
+      ${arm_or_thumb2_base_SOURCES}
+      )
+  endif()
+endif()
+
 set(arm_sync_SOURCES
   arm/sync_fetch_and_add_4.S
   arm/sync_fetch_and_add_8.S
@@ -456,6 +491,16 @@ set(thumb1_base_SOURCES
   ${GENERIC_SOURCES}
 )
 
+if(COMPILER_RT_ARM_OPTIMIZED_FP)
+  set(thumb1_base_SOURCES
+    arm/thumb1/mulsf3.S
+    arm/fnan2.c
+    arm/fnorm2.c
+    arm/funder.c
+    ${thumb1_base_SOURCES}
+  )
+endif()
+
 set(arm_EABI_RT_SOURCES
   arm/aeabi_cdcmp.S
   arm/aeabi_cdcmpeq_check_nan.c

compiler-rt/lib/builtins/arm/divsf3.S

@@ -0,0 +1,42 @@
+//===-- fnan2.c - Handle single-precision NaN inputs to binary operation --===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This helper function is available for use by single-precision float
+// arithmetic implementations to handle propagating NaNs from the input
+// operands to the output, in a way that matches Arm hardware FP.
+//
+// On input, a and b are floating-point numbers in IEEE 754 encoding, and at
+// least one of them must be a NaN. The return value is the correct output NaN.
+//
+// A signalling NaN in the input (with bit 22 clear) takes priority over any
+// quiet NaN, and is adjusted on return by setting bit 22 to make it quiet. If
+// both inputs are the same type of NaN then the first input takes priority:
+// the input a is used instead of b.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+uint32_t __compiler_rt_fnan2(uint32_t a, uint32_t b) {
+  // Make shifted-left copies of a and b to discard the sign bit. Then add 1 at
+  // the bit position where the quiet vs signalling bit ended up. This squashes
+  // all the signalling NaNs to the top of the range of 32-bit values, from
+  // 0xff800001 to 0xffffffff inclusive; meanwhile, all the quiet NaN values
+  // wrap round to the bottom, from 0 to 0x007fffff inclusive. So we can detect
+  // a signalling NaN by asking if it's greater than 0xff800000, and a quiet
+  // one by asking if it's less than 0x00800000.
+  uint32_t aadj = (a << 1) + 0x00800000;
+  uint32_t badj = (b << 1) + 0x00800000;
+  if (aadj > 0xff800000)   // a is a signalling NaN?
+    return a | 0x00400000; //   if so, return it with the quiet bit set
+  if (badj > 0xff800000)   // b is a signalling NaN?
+    return b | 0x00400000; //   if so, return it with the quiet bit set
+  if (aadj < 0x00800000)   // a is a quiet NaN?
+    return a;              // if so, return it
+  return b;                // otherwise we expect b must be a quiet NaN
+}

compiler-rt/lib/builtins/arm/fnan2.c

@@ -0,0 +1,62 @@
+//===-- fnorm2.c - Handle single-precision denormal inputs to binary op ---===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This helper function is available for use by single-precision float
+// arithmetic implementations, to handle denormal inputs on entry by
+// renormalizing the mantissa and modifying the exponent to match.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+// Structure containing the function's inputs and outputs.
+//
+// On entry: a, b are two input floating-point numbers, still in IEEE 754
+// encoding. expa and expb are the 8-bit exponents of those numbers, extracted
+// and shifted down to the low 8 bits of the word, with no other change.
+// Neither value should be zero, or have the maximum exponent (indicating an
+// infinity or NaN).
+//
+// On exit: each of a and b contains the mantissa of the input value, with the
+// leading 1 bit made explicit, and shifted up to the top of the word. If expa
+// was zero (indicating that a was denormal) then it is now represented as a
+// normalized number with an out-of-range exponent (zero or negative). The same
+// applies to expb and b.
+struct fnorm2 {
+  uint32_t a, b, expa, expb;
+};
+
+void __compiler_rt_fnorm2(struct fnorm2 *values) {
+  // Shift the mantissas of a and b to the right place to follow a leading 1 in
+  // the top bit, if there is one.
+  values->a <<= 8;
+  values->b <<= 8;
+
+  // Test if a is denormal.
+  if (values->expa == 0) {
+    // If so, decide how much further up to shift its mantissa, and adjust its
+    // exponent to match. This brings the leading 1 of the denormal mantissa to
+    // the top of values->a.
+    uint32_t shift = __builtin_clz(values->a);
+    values->a <<= shift;
+    values->expa = 1 - shift;
+  } else {
+    // Otherwise, leave the mantissa of a in its current position, and OR in
+    // the explicit leading 1.
+    values->a |= 0x80000000;
+  }
+
+  // Do the same operation on b.
+  if (values->expb == 0) {
+    uint32_t shift = __builtin_clz(values->b);
+    values->b <<= shift;
+    values->expb = 1 - shift;
+  } else {
+    values->b |= 0x80000000;
+  }
+}

compiler-rt/lib/builtins/arm/fnorm2.c

@@ -0,0 +1,78 @@
+//===-- funder.c - Handle single-precision floating-point underflow -------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This helper function is available for use by single-precision float
+// arithmetic implementations to handle underflowed output values, if they were
+// computed in the form of a normalized mantissa and an out-of-range exponent.
+//
+// On input: x should be a complete IEEE 754 floating-point value representing
+// the desired output scaled up by 2^192 (the same value that would have been
+// passed to an underflow trap handler in IEEE 754:1985).
+//
+// This isn't enough information to re-round to the correct output denormal
+// without also knowing whether x itself has already been rounded, and which
+// way. 'errsign' gives this information, by indicating the sign of the value
+// (true result - x). That is, if errsign > 0 it means the true value was
+// larger (x was rounded down); if errsign < 0 then x was rounded up; if
+// errsign == 0 then x represents the _exact_ desired output value.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+#define SIGNBIT 0x80000000
+#define MANTSIZE 23
+#define BIAS 0xc0
+
+uint32_t __compiler_rt_funder(uint32_t x, uint32_t errsign) {
+  uint32_t sign = x & SIGNBIT;
+  uint32_t exponent = (x << 1) >> 24;
+
+  // Rule out exponents so small (or large!) that no denormalisation
+  // is needed.
+  if (exponent > BIAS) {
+    // Exponent 0xc1 or above means a normalised number got here by
+    // mistake, so we just remove the 0xc0 exponent bias and go
+    // straight home.
+    return x - (BIAS << MANTSIZE);
+  }
+  uint32_t bits_lost = BIAS + 1 - exponent;
+  if (bits_lost > MANTSIZE + 1) {
+    // The implicit leading 1 of the intermediate value's mantissa is
+    // below the lowest mantissa bit of a denormal by at least 2 bits.
+    // Round down to 0 unconditionally.
+    return sign;
+  }
+
+  // Make the full mantissa (with leading bit) at the top of the word.
+  uint32_t mantissa = 0x80000000 | (x << 8);
+  // Adjust by 1 depending on the sign of the error.
+  mantissa -= errsign >> 31;
+  mantissa += (-errsign) >> 31;
+
+  // Shift down to the output position, keeping the bits shifted off.
+  uint32_t outmant, shifted_off;
+  if (bits_lost == MANTSIZE + 1) {
+    // Special case for the exponent where we have to shift the whole
+    // of 'mantissa' off the bottom of the word.
+    outmant = 0;
+    shifted_off = mantissa;
+  } else {
+    outmant = mantissa >> (8 + bits_lost);
+    shifted_off = mantissa << (32 - (8 + bits_lost));
+  }
+
+  // Re-round.
+  if (shifted_off >> 31) {
+    outmant++;
+    if (!(shifted_off << 1))
+      outmant &= ~1; // halfway case: round to even
+  }
+
+  return sign | outmant;
+}