Cranelift: ISLE mid-end performance regression (-9.20%)

[bongjunj]

Hi,

this is a follow-up of #12106 .

Although we've removed two sorts of performance regressing mid-end ISLE rules,
there still remains a significant performance degradation as well as other suspected cases.
(There is, of course, a bright side: we have significant performance improvements for many cases!)

Performance Regression:

• shootout-switch
• pulldown-cmark

First, here is the backing data of the performance regression:

Benchmark	No Opt	Main	Main Speedup
blake3-scalar	317,727	317,719	0.00%
blake3-simd	313,115	306,232	2.25%
bz2	87,201,400	86,337,330	1.00%
pulldown-cmark	6,580,174	6,905,992	-4.72%
regex	209,743,816	210,183,175	-0.21%
shootout-ackermann	8,498,140	7,764,439	9.45%
shootout-base64	381,721,177	352,724,661	8.22%
shootout-ctype	830,813,398	796,486,698	4.31%
shootout-ed25519	9,583,747,723	9,395,321,203	2.01%
shootout-fib2	3,009,269,670	3,010,509,565	-0.04%
shootout-gimli	5,338,258	5,401,697	-1.17%
shootout-heapsort	2,382,073,831	2,375,914,107	0.26%
shootout-keccak	25,168,386	21,112,482	19.21%
shootout-matrix	538,696,036	544,739,691	-1.11%
shootout-memmove	36,156,621	36,115,998	0.11%
shootout-minicsv	1,481,713,625	1,291,534,227	14.73%
shootout-nestedloop	449	442	1.43%
shootout-random	630,328,205	439,691,474	43.36%
shootout-ratelimit	39,148,817	39,956,714	-2.02%
shootout-seqhash	8,869,585,125	8,639,110,150	2.67%
shootout-sieve	905,404,028	840,777,681	7.69%
shootout-switch	139,525,474	153,663,682	-9.20%
shootout-xblabla20	2,891,404	2,907,369	-0.55%
shootout-xchacha20	4,384,703	4,395,319	-0.24%
spidermonkey	636,104,785	631,998,404	0.65%

Unlike the previous cases, the cause is not obvious.

19245 clif/v-no-opts/shootout-switch/wasm[0]--function[9]--__original_main.clif
19241 clif/v-main/shootout-switch/wasm[0]--function[9]--__original_main.clif

The number of instructions does not increase significantly from no-opt to main.
However, the applied optimizations make the program use long-lived value:

--- /data/bongjun/clif/v-no-opts/shootout-switch/wasm[0]--function[9]--__original_main.clif	2025-12-08 12:43:58.406738645 +0000
+++ /data/bongjun/clif/v-main/shootout-switch/wasm[0]--function[9]--__original_main.clif	2025-12-08 12:49:01.961085326 +0000

-                                    v8572 = iconst.i32 1066
-                                    v8573 = iconst.i32 0
-@d20b                               v4324 = call fn1(v0, v0, v8572, v8573)  ; v8572 = 1066, v8573 = 0
-                                    v8574 = iadd.i64 v105, v106  ; v106 = 3584
-@d219                               v4333 = load.i32 little heap v8574
-                                    v8575 = iconst.i32 6
-                                    v8576 = icmp uge v4333, v8575  ; v8575 = 6
+                                    v8603 = iconst.i32 1066
+                                    v8604 = iconst.i32 0
+@d20b                               v4324 = call fn1(v0, v0, v8603, v8604)  ; v8603 = 1066, v8604 = 0
+                                    v8605 = iadd.i64 v11, v106  ; v106 = 3584
+@d219                               v4333 = load.i32 little heap v8605
+                                    v8606 = iconst.i32 6
+                                    v8607 = icmp uge v4333, v8606  ; v8606 = 6

See v8574 and v8605 which uses v105 and v11.
v11 is defined at the beginning, but v105 is defined later than v11:

                                block0(v0: i64, v1: i64):
@01f0                               v5 = load.i32 notrap aligned table v0+256
@01f6                               v6 = iconst.i32 16
@01f8                               v7 = isub v5, v6  ; v6 = 16
@01fb                               store notrap aligned table v7, v0+256
@0203                               v9 = iconst.i32 0x2710
@0207                               v11 = load.i64 notrap aligned readonly can_move checked v0+56

...

@02d6                               v105 = iadd.i64 v11, v4439

This might increase the register pressure, causing more spills which can degrade the performance.