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cmov: use asm! barrier in portable backend (where stable) - #1334
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On targets where `asm!` is stable but we don't have a backend using architecture-specific predication instructions, uses `asm!` to insert an fence/barrier which should hopefully accomplish the intended goal as well or better than `core::hint::black_box`.
tarcieri
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Jan 14, 2026
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@NicsTr can you verify this still accomplishes the intended codegen? |
NicsTr
commented
Jan 14, 2026
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When testing .section .text.not_ct::test_ct_cmov,"ax",%progbits .globl not_ct::test_ct_cmov .p2align 1 .type not_ct::test_ct_cmov,%function .code 16 .thumb_funcnot_ct::test_ct_cmov: .fnstart .cfi_sections .debug_frame .cfi_startproc .save {r4, r5, r7, lr}push {r4, r5, r7, lr} .cfi_def_cfa_offset 16 .cfi_offset lr,-4 .cfi_offset r7,-8 .cfi_offset r5,-12 .cfi_offset r4,-16 .setfp r7,sp, #8add r7,sp, #8 .cfi_def_cfa r7,8 .pad #4subsp, #4 uxtb r3, r2 subs r2, r3, #1mov r4, r3 sbcs r4, r2 ldrb r2,[r0]mov r5,sp strb r4,[r5] @APP @ r5 @NO_APPcmp r3, #0 bne .LBB0_2mov r1, r2.LBB0_2: strb r1,[r0]addsp, #4pop {r4, r5, r7, pc}Is this approach of |
tarcieri
commented
Jan 14, 2026
MemberAuthor
That was the hypothesis but perhaps it isn't true |
tarcieri added a commit
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Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
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Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
tarcieri added a commit
that referenced
this pull request
Jan 15, 2026
In #1332 we ran into LLVM inserting branches in this routine for `thumbv6m-none-eabi` targets. It was "fixed" by fiddling around with `black_box` but that seems brittle. In #1334 we attempted a simple portable `asm!` optimization barrier approach but it did not work as expected. This instead opts to implement one of the fiddliest bits, mask generation, using ARM assembly instead. The resulting assembly is actually more efficient than what rustc/LLVM outputs and avoids touching the stack pointer. It's a simple enough function to implement in assembly on other platforms with stable `asm!` too, but this is a start.
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On targets where
asm!is stable but we don't have a backend using architecture-specific predication instructions, usesasm!to insert an fence/barrier which should hopefully accomplish the intended goal as well or better thancore::hint::black_box, using a similar approach to what was introduced inzeroizein #1252.Suggested by @newpavlov in #1332.