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Avoid using ebx as an asm! operand #185

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merged 3 commits into from
Oct 5, 2021
Merged

Avoid using ebx as an asm! operand #185

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8b8ccfb
Revert "counters: use AT&T inline asm syntax for older LLVM."
andjo403 Oct 3, 2021
326c6a4
Avoid using ebx as an asm! operand
andjo403 Oct 3, 2021
404e220
use functions to avoid duplicated asm
andjo403 Oct 3, 2021
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12 changes: 10 additions & 2 deletions measureme/src/counters.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -543,16 +543,20 @@ mod hw {
asm!(
// Dummy `cpuid(0)` to serialize instruction execution.
"xor eax, eax",
// LLVM sometimes reserves `ebx` for its internal use, we so we need to use
// a scratch register for it instead.
"mov {tmp_rbx:r}, rbx",
"cpuid",
"mov rbx, {tmp_rbx:r}",
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I've noticed this. This is frustrating, as it makes cpuid undesirably troublesome (I wish Intel and AMD would agree on e.g. mfence/lfence being serializing). I suspect that using a LLVM intrinsic for cpuid would result in shorter codegen, can you try this? Either way, can you PM me on Zulip?

I've let that rustc PR sit, sadly (long story short, other factors forced us to abandon the grant it was part of), and it would be good to take stock of what's useful and what's just hacks we only needed to get the results we wanted.


"mov ecx, {rdpmc_ecx:e}",
"rdpmc",
rdpmc_ecx = in(reg) reg_idx,
tmp_rbx = out(reg) _,
out("eax") lo,
out("edx") hi,

// `cpuid` clobbers (not overwritten by `rdpmc`).
out("ebx") _,
out("ecx") _,

options(nostack),
Expand All @@ -574,7 +578,11 @@ mod hw {
asm!(
// Dummy `cpuid(0)` to serialize instruction execution.
"xor eax, eax",
// LLVM sometimes reserves `ebx` for its internal use, we so we need to use
// a scratch register for it instead.
"mov {tmp_rbx:r}, rbx",
"cpuid",
"mov rbx, {tmp_rbx:r}",

"mov ecx, {a_rdpmc_ecx:e}",
"rdpmc",
Expand All @@ -586,11 +594,11 @@ mod hw {
a_rdpmc_eax = out(reg) a_lo,
a_rdpmc_edx = out(reg) a_hi,
b_rdpmc_ecx = in(reg) b_reg_idx,
tmp_rbx = out(reg) _,
out("eax") b_lo,
out("edx") b_hi,

// `cpuid` clobbers (not overwritten by `rdpmc`).
out("ebx") _,
out("ecx") _,

options(nostack),
Expand Down