Wrong argument register is used

Nobody has claimed this yet.

On a mips-elf target, (what I hope) equivalent C and Rust code produce different results, namely, the Rust version is trying to take the first argument from the second argument register

Build setup

FROM ubuntu:noble

ENV RUSTUP_HOME=/usr/local/rustup \
    CARGO_HOME=/usr/local/cargo \
    PATH=/usr/local/cargo/bin:$PATH \
    RUST_VERSION=nightly-2025-06-28-x86_64-unknown-linux-gnu \
    CCACHE_DIR=/w/ccache CC="ccache gcc" CXX="ccache g++"

RUN <<EOF
apt-get update
apt-get install -y build-essential libgmp-dev libmpfr-dev libmpc-dev libncurses5-dev flex bison autogen dejagnu git ccache wget

ccache --set-config max_size=0

# stolen from rust docker
wget https://static.rust-lang.org/rustup/archive/1.28.2/x86_64-unknown-linux-gnu/rustup-init
chmod +x ./rustup-init
./rustup-init -y --no-modify-path --profile complete --default-toolchain $RUST_VERSION --default-host x86_64-unknown-linux-gnu
chmod -R a+w $RUSTUP_HOME $CARGO_HOME
EOF

Compile binutils 2.44 with ./configure --disable-werror --target=mips-elf and install
Compile gccrs fcc21680857077d01a0d6b9336eea3999da351d1 with ../gccrs/configure --enable-languages=rust --disable-shared --disable-nls --target=mips-elf : (reconfigured) ../gccrs/configure --enable-languages=rust --disable-shared --disable-threads --disable-libmudflap --disable-libssp --disable-libstdcxx --disable-nls --disable-libgcc --disable-libgomp --disable-libatomic --target=mips-elf --without-headers and install

Compiling with -frust-incomplete-and-experimental-compiler-do-not-use -O2 -S

void start(unsigned char *ptr) {
    void call_with_a0(unsigned char *ptr);

    call_with_a0(ptr);
}

#[no_mangle]
pub extern "C" fn start(ptr: *mut u8) {
    extern "C" {
        fn call_with_a0(ptr: *mut u8);
    }

    unsafe {
        call_with_a0(ptr);
    }
}

Full output assembly

	.file	1 "a0.c"
	.section .mdebug.abi32
	.previous
	.nan	legacy
	.module	fp=32
	.module	nooddspreg
	.module	arch=mips1
	.text
	.align	2
	.globl	start
	.set	nomips16
	.set	nomicromips
	.ent	start
	.type	start, @function
start:
	.frame	$sp,0,$31		# vars= 0, regs= 0/0, args= 0, gp= 0
	.mask	0x00000000,0
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	j	call_with_a0
	nop

	.set	macro
	.set	reorder
	.end	start
	.size	start, .-start
	.ident	"GCC: (GNU) 16.0.0 20250604 (experimental)"

	.file	1 "a0.rs"
	.section .mdebug.abi32
	.previous
	.nan	legacy
	.module	fp=32
	.module	nooddspreg
	.module	arch=mips1
	.text
	.align	2
	.globl	start
$LFB0 = .
	.cfi_startproc
	.set	nomips16
	.set	nomicromips
	.ent	start
	.type	start, @function
start:
	.frame	$sp,24,$31		# vars= 0, regs= 1/0, args= 16, gp= 0
	.mask	0x80000000,-4
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	addiu	$sp,$sp,-24
	.cfi_def_cfa_offset 24
	sw	$4,24($sp)
	sw	$31,20($sp)
	.cfi_offset 31, -4
	jal	call_with_a0
	move	$4,$5                                                   # <---- notable: move $a0, $a1 ..?

	lw	$31,20($sp)
	lw	$2,24($sp)
	jr	$31
	addiu	$sp,$sp,24

	.cfi_def_cfa_offset 0
	.cfi_restore 31
	.set	macro
	.set	reorder
	.end	start
	.cfi_endproc
$LFE0:
	.size	start, .-start
	.ident	"GCC: (GNU) 16.0.0 20250604 (experimental)"

diff --git a/c.s b/r.s
index f27ce13..350c65d 100644
--- a/c.s
+++ b/r.s
@@ -1,4 +1,4 @@
-	.file	1 "a0.c"
+	.file	1 "a0.rs"
 	.section .mdebug.abi32
 	.previous
 	.nan	legacy
@@ -8,21 +8,37 @@
 	.text
 	.align	2
 	.globl	start
+$LFB0 = .
+	.cfi_startproc
 	.set	nomips16
 	.set	nomicromips
 	.ent	start
 	.type	start, @function
 start:
-	.frame	$sp,0,$31		# vars= 0, regs= 0/0, args= 0, gp= 0
-	.mask	0x00000000,0
+	.frame	$sp,24,$31		# vars= 0, regs= 1/0, args= 16, gp= 0
+	.mask	0x80000000,-4
 	.fmask	0x00000000,0
 	.set	noreorder
 	.set	nomacro
-	j	call_with_a0
-	nop
+	addiu	$sp,$sp,-24
+	.cfi_def_cfa_offset 24
+	sw	$4,24($sp)
+	sw	$31,20($sp)
+	.cfi_offset 31, -4
+	jal	call_with_a0
+	move	$4,$5                                                   # <---- notable: move $a0, $a1 ..?
 
+	lw	$31,20($sp)
+	lw	$2,24($sp)
+	jr	$31
+	addiu	$sp,$sp,24
+
+	.cfi_def_cfa_offset 0
+	.cfi_restore 31
 	.set	macro
 	.set	reorder
 	.end	start
+	.cfi_endproc
+$LFE0:
 	.size	start, .-start
 	.ident	"GCC: (GNU) 16.0.0 20250604 (experimental)"

EDIT 1: Switch target from mips-unknown-netbsd to mips-elf, same results

EDIT 2: Remove c_void, use u8 pointers

Start by reproducing the reduced C and Rust examples with gccrs on the mips-elf target using the shown build and compiler flags. Compare the generated a0.c and a0.rs assembly, focusing on the call to call_with_a0 and its argument registers. Done means the Rust output passes the pointer in the same register as the equivalent C output.

Written by the indexing model from the issue text.

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