YJIT: x86: Fix panic writing 32-bit number with top bit set

Previously, `asm.mov(m32, imm32)` panicked when `imm32 > 0x80000000`. It
attempted to split imm32 into a register before doing the store, but
then the register size didn't match the destination size.

Instead of splitting, use the `MOV r/m32, imm32` form which works for
all 32-bit values. Adjust asserts that assumed that all forms undergo
sign extension, which is not true for this case.

See: 54edc93

Author: XrXr

yjit/src/asm/x86_64/mod.rs

@@ -1027,7 +1027,10 @@ pub fn mov(cb: &mut CodeBlock, dst: X86Opnd, src: X86Opnd) {
             }
 
             let output_num_bits:u32 = if mem.num_bits > 32 { 32 } else { mem.num_bits.into() };
-            assert!(imm_num_bits(imm.value) <= (output_num_bits as u8));
+            assert!(
+                mem.num_bits < 64 || imm_num_bits(imm.value) <= (output_num_bits as u8),
+                "immediate value should be small enough to survive sign extension"
+            );
             cb.write_int(imm.value as u64, output_num_bits);
         },
         // M + UImm
@@ -1042,7 +1045,10 @@ pub fn mov(cb: &mut CodeBlock, dst: X86Opnd, src: X86Opnd) {
             }
 
             let output_num_bits = if mem.num_bits > 32 { 32 } else { mem.num_bits.into() };
-            assert!(imm_num_bits(uimm.value as i64) <= (output_num_bits as u8));
+            assert!(
+                mem.num_bits < 64 || imm_num_bits(uimm.value as i64) <= (output_num_bits as u8),
+                "immediate value should be small enough to survive sign extension"
+            );
             cb.write_int(uimm.value, output_num_bits);
         },
         // * + Imm/UImm

yjit/src/asm/x86_64/tests.rs

@@ -193,6 +193,7 @@ fn test_mov() {
     check_bytes("48c7470801000000", |cb| mov(cb, mem_opnd(64, RDI, 8), imm_opnd(1)));
     //check_bytes("67c7400411000000", |cb| mov(cb, mem_opnd(32, EAX, 4), imm_opnd(0x34))); // We don't distinguish between EAX and RAX here - that's probably fine?
     check_bytes("c7400411000000", |cb| mov(cb, mem_opnd(32, RAX, 4), imm_opnd(17)));
+    check_bytes("c7400401000080", |cb| mov(cb, mem_opnd(32, RAX, 4), uimm_opnd(0x80000001)));
     check_bytes("41895814", |cb| mov(cb, mem_opnd(32, R8, 20), EBX));
     check_bytes("4d8913", |cb| mov(cb, mem_opnd(64, R11, 0), R10));
     check_bytes("48c742f8f4ffffff", |cb| mov(cb, mem_opnd(64, RDX, -8), imm_opnd(-12)));

yjit/src/backend/x86_64/mod.rs

@@ -315,19 +315,24 @@ impl Assembler
                             let opnd1 = asm.load(*src);
                             asm.mov(*dest, opnd1);
                         },
-                        (Opnd::Mem(_), Opnd::UImm(value)) => {
-                            // 32-bit values will be sign-extended
-                            if imm_num_bits(*value as i64) > 32 {
+                        (Opnd::Mem(Mem { num_bits, .. }), Opnd::UImm(value)) => {
+                            // For 64 bit destinations, 32-bit values will be sign-extended
+                            if *num_bits == 64 && imm_num_bits(*value as i64) > 32 {
                                 let opnd1 = asm.load(*src);
                                 asm.mov(*dest, opnd1);
                             } else {
                                 asm.mov(*dest, *src);
                             }
                         },
-                        (Opnd::Mem(_), Opnd::Imm(value)) => {
-                            if imm_num_bits(*value) > 32 {
+                        (Opnd::Mem(Mem { num_bits, .. }), Opnd::Imm(value)) => {
+                            // For 64 bit destinations, 32-bit values will be sign-extended
+                            if *num_bits == 64 && imm_num_bits(*value) > 32 {
                                 let opnd1 = asm.load(*src);
                                 asm.mov(*dest, opnd1);
+                            } else if uimm_num_bits(*value as u64) <= *num_bits {
+                                // If the bit string is short enough for the destination, use the unsigned representation.
+                                // Note that 64-bit and negative values are ruled out.
+                                asm.mov(*dest, Opnd::UImm(*value as u64));
                             } else {
                                 asm.mov(*dest, *src);
                             }
@@ -1317,4 +1322,19 @@ mod tests {
             0x13: mov qword ptr [rbx], rax
         "});
     }
+
+    #[test]
+    fn test_mov_m32_imm32() {
+        let (mut asm, mut cb) = setup_asm();
+
+        let shape_opnd = Opnd::mem(32, C_RET_OPND, 0);
+        asm.mov(shape_opnd, Opnd::UImm(0x8000_0001));
+        asm.mov(shape_opnd, Opnd::Imm(0x8000_0001));
+
+        asm.compile_with_num_regs(&mut cb, 0);
+        assert_disasm!(cb, "c70001000080c70001000080", {"
+            0x0: mov dword ptr [rax], 0x80000001
+            0x6: mov dword ptr [rax], 0x80000001
+        "});
+    }
 }