cranelift: Add Bswap instruction (#1092) (#5147)

Adds Bswap to the Cranelift IR. Implements the Bswap instruction
in the x64 and aarch64 codegen backends. Cranelift users can now:
```
builder.ins().bswap(value)
```
to get a native byteswap instruction.

* x64: implements the 32- and 64-bit bswap instruction, following
the pattern set by similar unary instrutions (Neg and Not) - it
only operates on a dst register, but is parameterized with both
a src and dst which are expected to be the same register.

As x64 bswap instruction is only for 32- or 64-bit registers,
the 16-bit swap is implemented as a rotate left by 8.

Updated x64 RexFlags type to support emitting for single-operand
instructions like bswap

* aarch64: Bswap gets emitted as aarch64 rev16, rev32,
or rev64 instruction as appropriate.

* s390x: Bswap was already supported in backend, just had to add
a bit of plumbing

* For completeness, added bswap to the interpreter as well.

* added filetests and runtests for each ISA

* added bswap to fuzzgen, thanks to afonso360 for the code there

* 128-bit swaps are not yet implemented, that can be done later

cranelift/filetests/filetests/isa/aarch64/bswap.clif

@@ -0,0 +1,34 @@
+test compile precise-output
+set unwind_info=false
+target aarch64
+
+function %f0(i64) -> i64 {
+block0(v0: i64):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   rev64 x0, x0
+;   ret
+
+function %f1(i32) -> i32 {
+block0(v0: i32):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   rev32 w0, w0
+;   ret
+
+function %f2(i16) -> i16 {
+block0(v0: i16):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   rev16 w0, w0
+;   ret
+

cranelift/filetests/filetests/isa/s390x/bswap.clif

@@ -0,0 +1,34 @@
+test compile precise-output
+target s390x
+
+function %bswap_i64(i64) -> i64 {
+block0(v0: i64):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   lrvgr %r2, %r2
+;   br %r14
+
+function %bswap_i32(i32) -> i32 {
+block0(v0: i32):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   lrvr %r2, %r2
+;   br %r14
+
+function %bswap_i16(i16) -> i16 {
+block0(v0: i16):
+    v1 = bswap v0
+    return v1
+}
+
+; block0:
+;   lrvr %r5, %r2
+;   srlk %r2, %r5, 16
+;   br %r14
+

cranelift/filetests/filetests/isa/x64/bswap.clif

@@ -0,0 +1,48 @@
+test compile precise-output
+target x86_64
+
+function %f0(i64) -> i64 {
+block0(v0: i64):
+  v1 = bswap v0
+  return v1
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movq    %rdi, %rax
+;   bswapq  %rax, %rax
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %f1(i32) -> i32 {
+block0(v0: i32):
+  v1 = bswap v0
+  return v1
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movq    %rdi, %rax
+;   bswapl  %eax, %eax
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %f2(i16) -> i16 {
+block0(v0: i16):
+  v1 = bswap v0
+  return v1
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movq    %rdi, %rax
+;   rolw    $8, %ax, %ax
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+