targets/docs: Build with C++20

- Switch docs builds to C++20.
- Remove compound operations for volatile variables, which are
  deprecated in C++20.

Change-Id: Id60e758bc295b4271960c3d25a53e6ef82d5b2c6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/126911
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

5 files changed, 24 insertions, 12 deletions

diff --git a/pw_hdlc/size_report/hdlc_size_report.cc b/pw_hdlc/size_report/hdlc_size_report.cc
index 540d9de54..b8cf35e44 100644
--- a/pw_hdlc/size_report/hdlc_size_report.cc
+++ b/pw_hdlc/size_report/hdlc_size_report.cc
@@ -90,7 +90,7 @@ void HdlcSizeReport() {
   Result<hdlc::Frame> frame = decoder.Process(std::byte('~'));
   // Force use of nodiscard frame.
   if (frame.ok()) {
-    get_size++;
+    get_size = get_size + 1;
   }
   decoder.Clear();
 #endif
diff --git a/pw_sys_io_baremetal_stm32f429/sys_io_baremetal.cc b/pw_sys_io_baremetal_stm32f429/sys_io_baremetal.cc
index 4475a0851..7cd8fc594 100644
--- a/pw_sys_io_baremetal_stm32f429/sys_io_baremetal.cc
+++ b/pw_sys_io_baremetal_stm32f429/sys_io_baremetal.cc
@@ -134,24 +134,30 @@ volatile UsartBlock& usart1 =
 
 extern "C" void pw_sys_io_stm32f429_Init() {
   // Enable 'A' GIPO clocks.
-  platform_rcc.ahb1_config |= kGpioAEnable;
+  platform_rcc.ahb1_config = platform_rcc.ahb1_config | kGpioAEnable;
 
   // Enable Uart TX pin.
   // Output type defaults to push-pull (rather than open/drain).
-  gpio_a.modes |= kGpioPortModeAlternate << kGpio9PortModePos;
-  gpio_a.out_speed |= kGpioSpeedVeryHigh << kGpio9PortSpeedPos;
-  gpio_a.pull_up_down |= kPullTypePullUp << kGpio9PullTypePos;
-  gpio_a.alt_high |= kGpioAlternateFunctionUsart1 << kGpio9AltModeHighPos;
+  gpio_a.modes = gpio_a.modes | (kGpioPortModeAlternate << kGpio9PortModePos);
+  gpio_a.out_speed =
+      gpio_a.out_speed | (kGpioSpeedVeryHigh << kGpio9PortSpeedPos);
+  gpio_a.pull_up_down =
+      gpio_a.pull_up_down | (kPullTypePullUp << kGpio9PullTypePos);
+  gpio_a.alt_high =
+      gpio_a.alt_high | (kGpioAlternateFunctionUsart1 << kGpio9AltModeHighPos);
 
   // Enable Uart RX pin.
   // Output type defaults to push-pull (rather than open/drain).
-  gpio_a.modes |= kGpioPortModeAlternate << kGpio10PortModePos;
-  gpio_a.out_speed |= kGpioSpeedVeryHigh << kGpio10PortSpeedPos;
-  gpio_a.pull_up_down |= kPullTypePullUp << kGpio10PullTypePos;
-  gpio_a.alt_high |= kGpioAlternateFunctionUsart1 << kGpio10AltModeHighPos;
+  gpio_a.modes = gpio_a.modes | (kGpioPortModeAlternate << kGpio10PortModePos);
+  gpio_a.out_speed =
+      gpio_a.out_speed | (kGpioSpeedVeryHigh << kGpio10PortSpeedPos);
+  gpio_a.pull_up_down =
+      gpio_a.pull_up_down | (kPullTypePullUp << kGpio10PullTypePos);
+  gpio_a.alt_high =
+      gpio_a.alt_high | (kGpioAlternateFunctionUsart1 << kGpio10AltModeHighPos);
 
   // Initialize USART1. Initialized to 8N1 at the specified baud rate.
-  platform_rcc.apb2_config |= kUsart1Enable;
+  platform_rcc.apb2_config = platform_rcc.apb2_config | (kUsart1Enable);
 
   // Warning: Normally the baud rate register calculation is based off
   // peripheral 2 clock. For this code, the peripheral clock defaults to
diff --git a/targets/docs/BUILD.gn b/targets/docs/BUILD.gn
index 4e9b33c2e..10f341139 100644
--- a/targets/docs/BUILD.gn
+++ b/targets/docs/BUILD.gn
@@ -20,6 +20,7 @@ import("$dir_pw_protobuf_compiler/proto.gni")
 import("$dir_pw_third_party/nanopb/nanopb.gni")
 import("$dir_pw_toolchain/arm_gcc/toolchains.gni")
 import("$dir_pw_toolchain/generate_toolchain.gni")
+import("$dir_pw_toolchain/traits.gni")
 
 if (current_toolchain != default_toolchain) {
   pw_source_set("tokenized_log_handler") {
@@ -55,6 +56,8 @@ generate_toolchain("docs") {
     pw_log_BACKEND = dir_pw_log_tokenized
     pw_tokenizer_GLOBAL_HANDLER_WITH_PAYLOAD_BACKEND =
         get_path_info(":tokenized_log_handler", "abspath")
+
+    pw_toolchain_CXX_STANDARD = pw_toolchain_STANDARD.CXX20
   }
 }
 
diff --git a/targets/docs/target_docs.rst b/targets/docs/target_docs.rst
index 37db6c248..e130e8a17 100644
--- a/targets/docs/target_docs.rst
+++ b/targets/docs/target_docs.rst
@@ -6,6 +6,9 @@ docs
 The Pigweed docs target assembles Pigweed's reStructuredText and markdown
 documentation into a collection of HTML pages.
 
+The docs target uses the :ref:`target-stm32f429i-disc1` toolchain with C++20 for
+size reports.
+
 Building
 ========
 To build for this target, invoke ninja with the top-level "docs" group as the
diff --git a/targets/stm32f429i_disc1/boot.cc b/targets/stm32f429i_disc1/boot.cc
index 5150c7b5c..586159b13 100644
--- a/targets/stm32f429i_disc1/boot.cc
+++ b/targets/stm32f429i_disc1/boot.cc
@@ -34,7 +34,7 @@ void pw_boot_PreStaticMemoryInit() {
   // Memory mapped register to enable FPU. (ARMv7-M Section B3.2.2, Table B3-4)
   volatile uint32_t& arm_v7m_cpacr =
       *reinterpret_cast<volatile uint32_t*>(0xE000ED88u);
-  arm_v7m_cpacr |= kFpuEnableMask;
+  arm_v7m_cpacr = arm_v7m_cpacr | kFpuEnableMask;
 
   // Ensure the FPU configuration is committed and enabled before continuing and
   // potentially executing any FPU instructions, however rare that may be during