Don't limit ExternalName::TestName length (#4764)

In order to keep the `ExternalName` enum small, the `TestcaseName`
struct was limited to 17 bytes: a 1 byte length and a 16 byte buffer.
Due to alignment, that made `ExternalName` 20 bytes.

That fixed-size buffer means that the names of functions in Cranelift
filetests are truncated to fit, which limits our ability to give tests
meaningful names. And I think meaningful names are important in tests.

This patch replaces the inline `TestcaseName` buffer with a
heap-allocated slice. We don't care about performance for test names, so
an indirection out to the heap is fine in that case. But we do care
somewhat about the size of `ExternalName` when it's used during
compiles.

On 64-bit systems, `Box<[u8]>` is 16 bytes, so `TestcaseName` gets one
byte smaller. Unfortunately, its alignment is 8 bytes, so `ExternalName`
grows from 20 to 24 bytes.

According to `valgrind --tool=dhat`, this change has very little effect
on compiler performance. Building wasmtime with `--no-default-features
--release`, and compiling the pulldown-cmark benchmark from Sightglass,
I measured these differences between `main` and this patch:

- total number of allocations didn't change (`ExternalName::TestCase` is
  not used in normal compiles)
- 592 more bytes allocated over the process lifetime, out of 171.5MiB
- 320 more bytes allocated at peak heap size, out of 12MiB
- 0.24% more instructions executed
- 16,987 more bytes written
- 12,120 _fewer_ bytes read

cranelift/filetests/filetests/isa/aarch64/call-pauth.clif

@@ -14,7 +14,7 @@ block0(v0: i64):
 ;   stp fp, lr, [sp, #-16]!
 ;   mov fp, sp
 ; block0:
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   ldp fp, lr, [sp], #16
 ;   autiasp ; ret

cranelift/filetests/filetests/isa/aarch64/call.clif

@@ -14,7 +14,7 @@ block0(v0: i64):
 ;   stp fp, lr, [sp, #-16]!
 ;   mov fp, sp
 ; block0:
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -30,7 +30,7 @@ block0(v0: i32):
 ;   stp fp, lr, [sp, #-16]!
 ;   mov fp, sp
 ; block0:
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -54,7 +54,7 @@ block0(v0: i32):
 ;   stp fp, lr, [sp, #-16]!
 ;   mov fp, sp
 ; block0:
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -91,7 +91,7 @@ block0(v0: i8):
 ;   movz x6, #42
 ;   movz x7, #42
 ;   strb w15, [sp]
-;   ldr x15, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x15, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x15
 ;   add sp, sp, #16
 ;   virtual_sp_offset_adjust -16
@@ -140,25 +140,25 @@ block0:
 ;   mov fp, sp
 ;   sub sp, sp, #48
 ; block0:
-;   ldr x9, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 48, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x9, 8 ; b 12 ; data TestCase(%g0) + 0
 ;   blr x9
 ;   str q0, [sp]
-;   ldr x11, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 49, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x11, 8 ; b 12 ; data TestCase(%g1) + 0
 ;   blr x11
 ;   str q0, [sp, #16]
-;   ldr x13, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 49, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x13, 8 ; b 12 ; data TestCase(%g1) + 0
 ;   blr x13
 ;   str q0, [sp, #32]
-;   ldr x15, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x15, 8 ; b 12 ; data TestCase(%g2) + 0
 ;   blr x15
 ;   ldr q0, [sp]
-;   ldr x1, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 51, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x1, 8 ; b 12 ; data TestCase(%g3) + 0
 ;   blr x1
 ;   ldr q0, [sp, #16]
-;   ldr x3, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x3, 8 ; b 12 ; data TestCase(%g4) + 0
 ;   blr x3
 ;   ldr q0, [sp, #32]
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g4) + 0
 ;   blr x5
 ;   add sp, sp, #48
 ;   ldp fp, lr, [sp], #16
@@ -184,25 +184,25 @@ block0:
 ;   mov fp, sp
 ;   sub sp, sp, #48
 ; block0:
-;   ldr x9, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 48, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x9, 8 ; b 12 ; data TestCase(%g0) + 0
 ;   blr x9
 ;   str q0, [sp]
-;   ldr x11, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 48, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x11, 8 ; b 12 ; data TestCase(%g0) + 0
 ;   blr x11
 ;   str q0, [sp, #16]
-;   ldr x13, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 48, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x13, 8 ; b 12 ; data TestCase(%g0) + 0
 ;   blr x13
 ;   str q0, [sp, #32]
-;   ldr x15, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 49, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x15, 8 ; b 12 ; data TestCase(%g1) + 0
 ;   blr x15
 ;   ldr q0, [sp]
-;   ldr x1, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x1, 8 ; b 12 ; data TestCase(%g2) + 0
 ;   blr x1
 ;   ldr q0, [sp, #16]
-;   ldr x3, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x3, 8 ; b 12 ; data TestCase(%g2) + 0
 ;   blr x3
 ;   ldr q0, [sp, #32]
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g2) + 0
 ;   blr x5
 ;   add sp, sp, #48
 ;   ldp fp, lr, [sp], #16
@@ -232,25 +232,25 @@ block0:
 ;   mov fp, sp
 ;   sub sp, sp, #48
 ; block0:
-;   ldr x9, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 48, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x9, 8 ; b 12 ; data TestCase(%g0) + 0
 ;   blr x9
 ;   str q0, [sp]
-;   ldr x11, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 49, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x11, 8 ; b 12 ; data TestCase(%g1) + 0
 ;   blr x11
 ;   str q0, [sp, #16]
-;   ldr x13, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x13, 8 ; b 12 ; data TestCase(%g2) + 0
 ;   blr x13
 ;   str q0, [sp, #32]
-;   ldr x15, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 51, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x15, 8 ; b 12 ; data TestCase(%g3) + 0
 ;   blr x15
 ;   ldr q0, [sp]
-;   ldr x1, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x1, 8 ; b 12 ; data TestCase(%g4) + 0
 ;   blr x1
 ;   ldr q0, [sp, #16]
-;   ldr x3, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 53, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x3, 8 ; b 12 ; data TestCase(%g5) + 0
 ;   blr x3
 ;   ldr q0, [sp, #32]
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 2, ascii: [103, 54, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g6) + 0
 ;   blr x5
 ;   add sp, sp, #48
 ;   ldp fp, lr, [sp], #16
@@ -283,7 +283,7 @@ block0(v0: i64):
 ;   movz x0, #42
 ;   movz x2, #42
 ;   mov x1, x7
-;   ldr x10, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 49, 49, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x10, 8 ; b 12 ; data TestCase(%f11) + 0
 ;   blr x10
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -315,7 +315,7 @@ block0(v0: i64):
 ;   movz x3, #42
 ;   movz x0, #42
 ;   mov x2, x7
-;   ldr x10, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 49, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x10, 8 ; b 12 ; data TestCase(%f12) + 0
 ;   blr x10
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -347,7 +347,7 @@ block0(v0: i64):
 ;   movz x2, #42
 ;   movz x0, #42
 ;   mov x1, x7
-;   ldr x10, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 49, 51, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x10, 8 ; b 12 ; data TestCase(%f13) + 0
 ;   blr x10
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -388,7 +388,7 @@ block0(v0: i128, v1: i64):
 ;   mov x6, x14
 ;   str x13, [sp]
 ;   str x15, [sp, #8]
-;   ldr x7, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 49, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x7, 8 ; b 12 ; data TestCase(%f14) + 0
 ;   blr x7
 ;   add sp, sp, #16
 ;   virtual_sp_offset_adjust -16
@@ -431,7 +431,7 @@ block0(v0: i128, v1: i64):
 ;   mov x6, x14
 ;   str x13, [sp]
 ;   str x15, [sp, #8]
-;   ldr x7, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 49, 53, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x7, 8 ; b 12 ; data TestCase(%f15) + 0
 ;   blr x7
 ;   add sp, sp, #16
 ;   virtual_sp_offset_adjust -16
@@ -477,7 +477,7 @@ block0(v0: i64):
 ;   mov fp, sp
 ; block0:
 ;   mov x8, x0
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   mov x0, x8
 ;   ldp fp, lr, [sp], #16
@@ -496,7 +496,7 @@ block0(v0: i64):
 ;   str x24, [sp, #-16]!
 ; block0:
 ;   mov x24, x8
-;   ldr x5, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [103, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x5, 8 ; b 12 ; data TestCase(%g) + 0
 ;   blr x5
 ;   mov x8, x24
 ;   ldr x24, [sp], #16

cranelift/filetests/filetests/isa/aarch64/reftypes.clif

@@ -69,7 +69,7 @@ block3(v7: r64, v8: r64):
 ; block0:
 ;   str x1, [sp, #16]
 ;   str x0, [sp, #8]
-;   ldr x3, 8 ; b 12 ; data TestCase(TestcaseName { length: 1, ascii: [102, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x3, 8 ; b 12 ; data TestCase(%f) + 0
 ;   blr x3
 ;   mov x9, sp
 ;   ldr x11, [sp, #8]

cranelift/filetests/filetests/isa/aarch64/stack-limit.clif

@@ -42,7 +42,7 @@ block0(v0: i64):
 ;   subs xzr, sp, x0, UXTX
 ;   b.hs 8 ; udf
 ; block0:
-;   ldr x2, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 111, 111, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x2, 8 ; b 12 ; data TestCase(%foo) + 0
 ;   blr x2
 ;   ldp fp, lr, [sp], #16
 ;   ret
@@ -65,7 +65,7 @@ block0(v0: i64):
 ;   subs xzr, sp, x16, UXTX
 ;   b.hs 8 ; udf
 ; block0:
-;   ldr x2, 8 ; b 12 ; data TestCase(TestcaseName { length: 3, ascii: [102, 111, 111, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x2, 8 ; b 12 ; data TestCase(%foo) + 0
 ;   blr x2
 ;   ldp fp, lr, [sp], #16
 ;   ret

cranelift/filetests/filetests/isa/aarch64/symbol-value.clif

@@ -11,6 +11,6 @@ block0:
 }
 
 ; block0:
-;   ldr x0, 8 ; b 12 ; data TestCase(TestcaseName { length: 9, ascii: [109, 121, 95, 103, 108, 111, 98, 97, 108, 0, 0, 0, 0, 0, 0, 0] }) + 0
+;   ldr x0, 8 ; b 12 ; data TestCase(%my_global) + 0
 ;   ret