Re-enable FastISel for MIPS O32 with LLVM 20

[alexrp]

Zig Version

93cb44c

Steps to Reproduce and Observed Behavior

zig build test -fqemu --glibc-runtimes <...> test-behavior -Dtest-slow-targets -Dtest-target-filter=mips-linux.4.19...6.10.3-gnueabihf.2.28

Program received signal SIGBUS, Bus error.
0x00068bd8 in mem.eqlBytes (a=..., b=...) at /home/alexrp/Source/zig/lib/std/mem.zig:685
685                 x |= @as(u32, @bitCast(a[n..][0..4].*)) ^ @as(u32, @bitCast(b[n..][0..4].*));
(gdb) bt
#0  0x00068bd8 in mem.eqlBytes (a=..., b=...) at /home/alexrp/Source/zig/lib/std/mem.zig:685
#1  0x00063748 in mem.eql__anon_1648 (a=..., b=...) at /home/alexrp/Source/zig/lib/std/mem.zig:660
#2  0x0006392c in mem.startsWith__anon_1657 (haystack=..., needle=...) at /home/alexrp/Source/zig/lib/std/mem.zig:2913
#3  0x00055f10 in test_runner.main () at /home/alexrp/Source/zig/lib/compiler/test_runner.zig:38
#4  0x00055674 in start.callMain () at /home/alexrp/Source/zig/lib/std/start.zig:605
#5  start.callMainWithArgs () at /home/alexrp/Source/zig/lib/std/start.zig:574
#6  start.main (c_argc=3, c_argv=0x2b2ab314, c_envp=0x2b2ab324) at /home/alexrp/Source/zig/lib/std/start.zig:589
(gdb) disas $pc-100,$pc+100
Dump of assembler code from 0x68b74 to 0x68c3c:
   0x00068b74 <mem.eqlBytes+2068>:      sw      at,116(s8)
   0x00068b78 <mem.eqlBytes+2072>:      lw      v1,352(s8)
   0x00068b7c <mem.eqlBytes+2076>:      addu    v1,v1,v0
   0x00068b80 <mem.eqlBytes+2080>:      sw      v1,120(s8)
   0x00068b84 <mem.eqlBytes+2084>:      li      v1,4
   0x00068b88 <mem.eqlBytes+2088>:      addu    v0,v0,v1
   0x00068b8c <mem.eqlBytes+2092>:      sw      v0,124(s8)
   0x00068b90 <mem.eqlBytes+2096>:      sltu    at,at,v0
   0x00068b94 <mem.eqlBytes+2100>:      xori    at,at,0x1
   0x00068b98 <mem.eqlBytes+2104>:      bgtz    at,0x68bf0 <mem.eqlBytes+2192>
   0x00068b9c <mem.eqlBytes+2108>:      nop
   0x00068ba0 <mem.eqlBytes+2112>:      b       0x68bf8 <mem.eqlBytes+2200>
   0x00068ba4 <mem.eqlBytes+2116>:      nop
   0x00068ba8 <mem.eqlBytes+2120>:      b       0x68b4c <mem.eqlBytes+2028>
   0x00068bac <mem.eqlBytes+2124>:      nop
   0x00068bb0 <mem.eqlBytes+2128>:      lw      a1,136(s8)
   0x00068bb4 <mem.eqlBytes+2132>:      lw      a0,144(s8)
   0x00068bb8 <mem.eqlBytes+2136>:      lw      at,220(s8)
   0x00068bbc <mem.eqlBytes+2140>:      lw      at,-32728(at)
   0x00068bc0 <mem.eqlBytes+2144>:      addiu   t9,at,22496
   0x00068bc4 <mem.eqlBytes+2148>:      bal     0x557e0 <builtin.panicOutOfBounds>
   0x00068bc8 <mem.eqlBytes+2152>:      nop
   0x00068bcc <mem.eqlBytes+2156>:      lw      at,132(s8)
   0x00068bd0 <mem.eqlBytes+2160>:      lw      v0,112(s8)
   0x00068bd4 <mem.eqlBytes+2164>:      lw      v1,120(s8)
=> 0x00068bd8 <mem.eqlBytes+2168>:      lw      v1,0(v1)
   0x00068bdc <mem.eqlBytes+2172>:      xor     v0,v0,v1
   0x00068be0 <mem.eqlBytes+2176>:      or      at,at,v0
   0x00068be4 <mem.eqlBytes+2180>:      sw      at,320(s8)
   0x00068be8 <mem.eqlBytes+2184>:      b       0x68ac0 <mem.eqlBytes+1888>
   0x00068bec <mem.eqlBytes+2188>:      nop
   0x00068bf0 <mem.eqlBytes+2192>:      b       0x68bcc <mem.eqlBytes+2156>
   0x00068bf4 <mem.eqlBytes+2196>:      nop
   0x00068bf8 <mem.eqlBytes+2200>:      lw      a1,116(s8)
   0x00068bfc <mem.eqlBytes+2204>:      lw      a0,124(s8)
   0x00068c00 <mem.eqlBytes+2208>:      lw      at,220(s8)
   0x00068c04 <mem.eqlBytes+2212>:      lw      at,-32728(at)
   0x00068c08 <mem.eqlBytes+2216>:      addiu   t9,at,22496
   0x00068c0c <mem.eqlBytes+2220>:      bal     0x557e0 <builtin.panicOutOfBounds>
   0x00068c10 <mem.eqlBytes+2224>:      nop
   0x00068c14 <mem.eqlBytes+2228>:      li      at,2
   0x00068c18 <mem.eqlBytes+2232>:      sw      at,268(s8)
   0x00068c1c <mem.eqlBytes+2236>:      li      at,3
   0x00068c20 <mem.eqlBytes+2240>:      sw      at,272(s8)
   0x00068c24 <mem.eqlBytes+2244>:      li      at,4
   0x00068c28 <mem.eqlBytes+2248>:      sw      at,276(s8)
   0x00068c2c <mem.eqlBytes+2252>:      li      at,5
   0x00068c30 <mem.eqlBytes+2256>:      sw      at,280(s8)
   0x00068c34 <mem.eqlBytes+2260>:      lw      at,224(s8)
   0x00068c38 <mem.eqlBytes+2264>:      li      v0,0
End of assembler dump.

It looks like an LLVM miscompilation, but not sure yet.

Very oddly, -musleabihf is fine (but note that I haven't upstreamed my branch for that yet).

Expected Behavior

No failure.

[alexrp]

llvm/llvm-project#106231

[alexrp]

Workaround added in #21224; this issue now tracks removing the workaround with LLVM 20.