Auto merge of rust-lang#130987 - thejpster:revise-arm-platform-notes-soft-float, r=ehuss

Revise arm platform notes regarding soft float

This PR updates the Arm microcontroller platform docs to recommend `-fpregs` instead of `+soft-float` as [discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.60-Ctarget-feature.3D.2Bsoft-float.60.20considered.20harmful)

Author: bors

src/doc/rustc/src/platform-support/arm-none-eabi.md

@@ -42,16 +42,15 @@ their own document.
 There are two 32-bit instruction set architectures (ISAs) defined by Arm:
 
 - The [*A32 ISA*][a32-isa], with fixed-width 32-bit instructions. Previously
-  known as the *Arm* ISA, this originated with the original Arm1 of 1985 and has
+  known as the *Arm* ISA, this originated with the original ARM1 of 1985 and has
   been updated by various revisions to the architecture specifications ever
   since.
 - The [*T32 ISA*][t32-isa], with a mix of 16-bit and 32-bit width instructions.
   Note that this term includes both the original 16-bit width *Thumb* ISA
   introduced with the Armv4T architecture in 1994, and the later 16/32-bit sized
-  *Thumb-2* ISA introduced with the Armv6T2 architecture in 2003.
-
-Again, these ISAs have been revised by subsequent revisions to the relevant Arm
-architecture specifications.
+  *Thumb-2* ISA introduced with the Armv6T2 architecture in 2003. Again, these
+  ISAs have been revised by subsequent revisions to the relevant Arm
+  architecture specifications.
 
 There is also a 64-bit ISA with fixed-width 32-bit instructions called the *A64
 ISA*, but targets which implement that instruction set generally start with
@@ -106,10 +105,14 @@ features you do not have available, leaving you with the optimized instruction
 scheduling and support for the features you do have. More details are available
 in the detailed target-specific documentation.
 
-**Note:** Many target-features are currently unstable and subject to change, and
+<div class="warning">
+
+Many target-features are currently unstable and subject to change, and
 if you use them you should disassemble the compiler output and manually inspect
 it to ensure only appropriate instructions for your CPU have been generated.
 
+</div>
+
 If you wish to use the *target-cpu* and *target-feature* options, you can add
 them to your `.cargo/config.toml` file alongside any other flags your project
 uses (likely linker related ones):

src/doc/rustc/src/platform-support/thumbv7em-none-eabi.md

@@ -35,9 +35,9 @@ to use these flags.
 | CPU        | FPU | DSP | Target CPU  | Target Features |
 | ---------- | --- | --- | ----------- | --------------- |
 | Any        | No  | Yes | None        | None            |
-| Cortex-M4  | No  | Yes | `cortex-m4` | `+soft-float`   |
+| Cortex-M4  | No  | Yes | `cortex-m4` | `-fpregs`       |
 | Cortex-M4F | SP  | Yes | `cortex-m4` | None            |
-| Cortex-M7  | No  | Yes | `cortex-m7` | `+soft-float`   |
+| Cortex-M7  | No  | Yes | `cortex-m7` | `-fpregs`       |
 | Cortex-M7F | SP  | Yes | `cortex-m7` | `-fp64`         |
 | Cortex-M7F | DP  | Yes | `cortex-m7` | None            |
 
@@ -50,6 +50,13 @@ to use these flags.
 | Cortex-M7F | SP  | Yes | `cortex-m7` | `-fp64`         |
 | Cortex-M7F | DP  | Yes | `cortex-m7` | None            |
 
+<div class="warning">
+
+Never use the `-fpregs` *target-feature* with the `thumbv7em-none-eabihf` target
+as it will cause compilation units to have different ABIs, which is unsound.
+
+</div>
+
 ### Arm Cortex-M4 and Arm Cortex-M4F
 
 The target CPU is `cortex-m4`.
@@ -59,7 +66,7 @@ The target CPU is `cortex-m4`.
   * enabled by default with this *target*
 * Cortex-M4F has a single precision FPU
   * support is enabled by default with this *target-cpu*
-  * disable support using the `+soft-float` feature (`eabi` only)
+  * disable support using the `-fpregs` *target-feature* (`eabi` only)
 
 ### Arm Cortex-M7 and Arm Cortex-M7F
 
@@ -71,4 +78,4 @@ The target CPU is `cortex-m7`.
 * Cortex-M7F have either a single-precision or double-precision FPU
   * double-precision support is enabled by default with this *target-cpu*
     * opt-out by using the `-f64` *target-feature*
-  * disable support entirely using the `+soft-float` feature (`eabi` only)
+  * disable support entirely using the `-fpregs` *target-feature* (`eabi` only)

src/doc/rustc/src/platform-support/thumbv8m.main-none-eabi.md

@@ -39,22 +39,22 @@ to use these flags.
 | CPU         | FPU | DSP | MVE       | Target CPU    | Target Features       |
 | ----------- | --- | --- | --------- | ------------- | --------------------- |
 | Unspecified | No  | No  | No        | None          | None                  |
-| Cortex-M33  | No  | No  | No        | `cortex-m33`  | `+soft-float,-dsp`    |
-| Cortex-M33  | No  | Yes | No        | `cortex-m33`  | `+soft-float`         |
+| Cortex-M33  | No  | No  | No        | `cortex-m33`  | `-fpregs,-dsp`        |
+| Cortex-M33  | No  | Yes | No        | `cortex-m33`  | `-fpregs`             |
 | Cortex-M33  | SP  | No  | No        | `cortex-m33`  | `-dsp`                |
 | Cortex-M33  | SP  | Yes | No        | `cortex-m33`  | None                  |
-| Cortex-M35P | No  | No  | No        | `cortex-m35p` | `+soft-float,-dsp`    |
-| Cortex-M35P | No  | Yes | No        | `cortex-m35p` | `+soft-float`         |
+| Cortex-M35P | No  | No  | No        | `cortex-m35p` | `-fpregs,-dsp`        |
+| Cortex-M35P | No  | Yes | No        | `cortex-m35p` | `-fpregs`             |
 | Cortex-M35P | SP  | No  | No        | `cortex-m35p` | `-dsp`                |
 | Cortex-M35P | SP  | Yes | No        | `cortex-m35p` | None                  |
-| Cortex-M55  | No  | Yes | No        | `cortex-m55`  | `+soft-float,-mve`    |
+| Cortex-M55  | No  | Yes | No        | `cortex-m55`  | `-fpregs,-mve`        |
 | Cortex-M55  | DP  | Yes | No        | `cortex-m55`  | `-mve`                |
-| Cortex-M55  | No  | Yes | Int       | `cortex-m55`  | `+soft-float,-mve.fp` |
+| Cortex-M55  | No  | Yes | Int       | `cortex-m55`  | `-fpregs,-mve.fp,+mve`|
 | Cortex-M55  | DP  | Yes | Int       | `cortex-m55`  | `-mve.fp`             |
 | Cortex-M55  | DP  | Yes | Int+Float | `cortex-m55`  | None                  |
-| Cortex-M85  | No  | Yes | No        | `cortex-m85`  | `+soft-float,-mve`    |
+| Cortex-M85  | No  | Yes | No        | `cortex-m85`  | `-fpregs,-mve`        |
 | Cortex-M85  | DP  | Yes | No        | `cortex-m85`  | `-mve`                |
-| Cortex-M85  | No  | Yes | Int       | `cortex-m85`  | `+soft-float,-mve.fp` |
+| Cortex-M85  | No  | Yes | Int       | `cortex-m85`  | `-fpregs,-mve.fp,+mve`|
 | Cortex-M85  | DP  | Yes | Int       | `cortex-m85`  | `-mve.fp`             |
 | Cortex-M85  | DP  | Yes | Int+Float | `cortex-m85`  | None                  |
 
@@ -74,6 +74,19 @@ to use these flags.
 | Cortex-M85  | DP  | Yes | Int       | `cortex-m85`  | `-mve.fp`             |
 | Cortex-M85  | DP  | Yes | Int+Float | `cortex-m85`  | None                  |
 
+*Technically* you can use this hard-float ABI on a CPU which has no FPU but does
+have Integer MVE, because MVE provides the same set of registers as the FPU
+(including `s0` and `d0`). The particular set of flags that might enable this
+unusual scenario are currently not recorded here.
+
+<div class="warning">
+
+Never use the `-fpregs` *target-feature* with the `thumbv8m.main-none-eabihf`
+target as it will cause compilation units to have different ABIs, which is
+unsound.
+
+</div>
+
 ### Arm Cortex-M33
 
 The target CPU is `cortex-m33`.
@@ -83,7 +96,7 @@ The target CPU is `cortex-m33`.
   * enabled by default with this *target-cpu*
 * Has an optional single precision FPU
   * support is enabled by default with this *target-cpu*
-  * disable support using the `+soft-float` feature (`eabi` only)
+  * disable support using the `-fpregs` *target-feature* (`eabi` only)
 
 ### Arm Cortex-M35P
 
@@ -94,7 +107,7 @@ The target CPU is `cortex-m35p`.
   * enabled by default with this *target-cpu*
 * Has an optional single precision FPU
   * support is enabled by default with this *target-cpu*
-  * disable support using the `+soft-float` feature (`eabi` only)
+  * disable support using the `-fpregs` *target-feature* (`eabi` only)
 
 ### Arm Cortex-M55
 
@@ -106,7 +119,7 @@ The target CPU is `cortex-m55`.
 * Has an optional double-precision FPU that also supports half-precision FP16
   values
   * support is enabled by default with this *target-cpu*
-  * disable support using the `+soft-float` feature (`eabi` only)
+  * disable support using the `-fpregs` *target-feature* (`eabi` only)
 * Has optional support for M-Profile Vector Extensions
   * Also known as *Helium Technology*
   * Available with only integer support, or both integer/float support
@@ -125,7 +138,7 @@ The target CPU is `cortex-m85`.
 * Has an optional double-precision FPU that also supports half-precision FP16
   values
   * support is enabled by default with this *target-cpu*
-  * disable support using the `+soft-float` feature (`eabi` only)
+  * disable support using the `-fpregs` *target-feature* (`eabi` only)
 * Has optional support for M-Profile Vector Extensions
   * Also known as *Helium Technology*
   * Available with only integer support, or both integer/float support