[Bug target/127086] RISC-V: mode-switching fails to hoist VXRM writes out of some vectorized loops

[anton at ozlabs dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127086

--- Comment #4 from Anton Blanchard <anton at ozlabs dot org> ---
Thanks Jeff. I found this while chasing a 10+% regression with x264_r on
Ascalon when building with -flto. Looking at the instruction trace in a cycle
accurate simulator, much of the time was in this loop:

 csrrwi   x0, vxrm, 0x0
 vle8.v v2, (x12)
 vle8.v v1, (x10)
 vaaddu.vv v1, v1, v2
 vse8.v v1, (x11)
 addi     x11, x11, 16
 add      x12, x12, x13
 add      x10, x10, x13
 add      x16, x16, x29
 bne      x11, x23, . - 0x30
 add      x17, x28, x16
 sltiu    x17, x17, 31
 bne      x17, x0, . + 0x378
 sltiu    x17, x16, 31
 bne      x17, x0, . + 0x370

 csrrwi   x0, vxrm, 0x0
 vle8.v v2, (x12)
 vle8.v v1, (x10)
 vaaddu.vv v1, v1, v2
 vse8.v v1, (x11)
 addi     x11, x11, 16
 add      x12, x12, x13
 add      x10, x10, x13
 add      x16, x16, x29
 bne      x11, x23, . - 0x30
 add      x17, x28, x16
 sltiu    x17, x17, 31
 bne      x17, x0, . + 0x378
 sltiu    x17, x16, 31
 bne      x17, x0, . + 0x370

 csrrwi   x0, vxrm, 0x0
 vle8.v v2, (x12)
 vle8.v v1, (x10)
 vaaddu.vv v1, v1, v2
 vse8.v v1, (x11)
 addi     x11, x11, 16
 add      x12, x12, x13
 add      x10, x10, x13
 add      x16, x16, x29
 bne      x11, x23, . - 0x30
 add      x17, x28, x16
 sltiu    x17, x17, 31
 bne      x17, x0, . + 0x378
 sltiu    x17, x16, 31
 bne      x17, x0, . + 0x370

 csrrwi   x0, vxrm, 0x0

So the VXRM write ended up in the core loop. I wonder if -flto inlined enough
code to trip up on the LCM limitations.