Remove remaining traces of AT&T assembly syntax by TDecking · Pull Request #911 · rust-lang/compiler-builtins

TDecking · 2025-05-08T07:28:00Z

The current minimum LLVM version seems to be 18, way larger than required for resolving this FIXME.

tgross35 · 2025-05-08T08:02:46Z

compiler-builtins/src/mem/x86_64.rs

-    core::arch::asm!(
-        "repe movsb (%rsi), (%rdi)",
+    asm!(
+        "repe movsb [rdi], rsi",


In the original asm both rsi and rdi are dereferenced, here it’s only rdi. Is this still equivalent for some reason?

I don't think it is. I fixed this.

quaternic · 2025-05-10T14:22:08Z

compiler-builtins/src/mem/x86_64.rs

-    core::arch::asm!(
-        "repe movsb (%rsi), (%rdi)",
+    asm!(
+        "repe movsb [rdi], [rsi]",


Suggested change

"repe movsb [rdi], [rsi]",

"rep movsb [rdi], [rsi]",

The repe mnemonic is for "repeat while equal" and only makes sense for the string comparison operations
https://www.felixcloutier.com/x86/rep:repe:repz:repne:repnz
(but the encoding is the same so I guess assemblers don't care)

quaternic · 2025-05-10T14:36:01Z

compiler-builtins/src/mem/x86_64.rs

-    core::arch::asm!(
-        "repe stosb %al, (%rdi)",
+    asm!(
+        "repe stosb [rdi], al",


Suggested change

"repe stosb [rdi], al",

"rep stosb [rdi], al",

quaternic · 2025-05-10T15:08:59Z

compiler-builtins/src/mem/x86_64.rs

        inout("rsi") src.add(count - 1) => _,
        // We modify flags, but we restore it afterwards
-        options(att_syntax, nostack, preserves_flags)
+        options(nostack, preserves_flags)


This definitely shouldn't have preserves_flags. Flags are modified by each of add,sub,test. The comment refers to how std/cld are used to set/clear the direction flag, but that's separate since it must be cleared before exiting the inline assembly.

Actually, what is the point of the test instruction here? It only affects flags, and none of the subsequent instructions depend on any flags.

I think it can be removed: ae557bd had an early exit but ef37a23 removed it.

This diff gave me a similar question about

compiler-builtins/compiler-builtins/src/x86_64.rs

Lines 20 to 26 in e1d8a49

"2:",

"sub $0x1000,%rcx",

"test %rcx,(%rcx)",

"sub $0x1000,%rax",

"cmp $0x1000,%rax",

"ja 2b",

"1:",

. Aren't flags set by test overwritten by the sub then cmp before ja checks them?

Yeah, that looked weird at first, but I think the test there just used as a way to load from memory without modifying any registers (except flags). So the loop is just reading from memory at 4kB intervals for the side-effects only, which is exactly what a chkstk should be doing.

tgross35 · 2025-12-05T11:20:06Z

compiler-builtins/src/x86.rs

-            "test   %ecx,(%ecx)",
-            "lea    4(%esp),%eax",  // load pointer to the return address into eax
-            "mov    %ecx,%esp",     // install the new top of stack pointer into esp
-            "mov    -4(%eax),%ecx", // restore ecx
-            "push   (%eax)",        // push return address onto the stack
-            "sub    %esp,%eax",     // restore the original value in eax
+            "3:",
+            "sub    ecx, eax",
+            "test   [ecx], ecx",
+            "lea    eax, [esp + 4]",  // load pointer to the return address into eax
+            "mov    esp, ecx",     // install the new top of stack pointer into esp
+            "mov    ecx, [eax - 4]", // restore ecx
+            "push   [eax]",        // push return address onto the stack
+            "sub    eax, esp",     // restore the original value in eax


Nit: comments used to be aligned

tgross35 · 2025-12-05T11:21:37Z

@quaternic the issues you mentioned seem to be preexisting. Would you mind putting up a PR to fix them? We can do that independent of the syntax swap.

Then @TDecking you won't have to make any changes aside from resolving conflicts. Would you be able to rebase?

quaternic · 2025-12-05T13:30:35Z

@tgross35 I can, but I'd much prefer doing that in intel syntax, so merging this with those preexisting issues would be fine.

rustbot · 2025-12-05T14:03:48Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

TDecking · 2025-12-05T14:08:41Z

I've rebased and removed the final instances of att_syntax in probestack.rs.

tgross35

LGTM as a 1:1 change, thanks for the updates!

* `repe` is "repeat while equal", which only makes sense for string comparisons. Change it to `rep`. (The encoding is the same so there is no performance change.) * Remove an unneeded `test`. This was added in ae557bd ("Skip rep movsb in copy_backward if possible"). The `jz` was removed in ef37a23 ("Remove branches around rep movsb/stosb") but the `test` was missed. * Remove an incorrect `preserves_flags`; `add` and `sub` affect flags. Discussion: #911 Fixes: ef37a23 ("Remove branches around rep movsb/stosb") Fixes: c30322a ("Align destination in mem* instructions.") [ Added details to the commit message - Trevor ]

* `repe` is "repeat while equal", which only makes sense for string comparisons. Change it to `rep`. (The encoding is the same so there is no performance change.) * Remove an unneeded `test`. This was added in ae557bde4efc ("Skip rep movsb in copy_backward if possible"). The `jz` was removed in ef37a23d8417 ("Remove branches around rep movsb/stosb") but the `test` was missed. * Remove an incorrect `preserves_flags`; `add` and `sub` affect flags. Discussion: rust-lang/compiler-builtins#911 Fixes: ef37a23d8417 ("Remove branches around rep movsb/stosb") Fixes: c30322aafc9c ("Align destination in mem* instructions.") [ Added details to the commit message - Trevor ]

* `repe` is "repeat while equal", which only makes sense for string comparisons. Change it to `rep`. (The encoding is the same so there is no performance change.) * Remove an unneeded `test`. This was added in ae557bd ("Skip rep movsb in copy_backward if possible"). The `jz` was removed in ef37a23 ("Remove branches around rep movsb/stosb") but the `test` was missed. * Remove an incorrect `preserves_flags`; `add` and `sub` affect flags. Discussion: #911 Fixes: ef37a23 ("Remove branches around rep movsb/stosb") Fixes: c30322a ("Align destination in mem* instructions.") [ Added details to the commit message - Trevor ]

tgross35 reviewed May 8, 2025

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TDecking force-pushed the att-syntax branch from e4fee1c to 275acf2 Compare May 8, 2025 09:31

quaternic suggested changes May 10, 2025

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tgross35 reviewed Dec 5, 2025

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Remove remaining traces of AT&T assembly syntax

7980cfa

TDecking force-pushed the att-syntax branch from ee144cf to 7980cfa Compare December 5, 2025 14:03

TDecking requested a review from tgross35 December 5, 2025 16:31

tgross35 approved these changes Dec 5, 2025

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tgross35 merged commit f867373 into rust-lang:main Dec 5, 2025
38 checks passed

TDecking deleted the att-syntax branch December 5, 2025 19:25

quaternic mentioned this pull request Dec 6, 2025

remove incorrectly placed preserves_flags, and other inline asm fixes #1036

Merged

	"2:",
	"sub $0x1000,%rcx",
	"test %rcx,(%rcx)",
	"sub $0x1000,%rax",
	"cmp $0x1000,%rax",
	"ja 2b",
	"1:",

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TDecking commented May 8, 2025

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quaternic Dec 5, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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tgross35 commented Dec 5, 2025

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quaternic commented Dec 5, 2025

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rustbot commented Dec 5, 2025

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TDecking commented Dec 5, 2025

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quaternic Dec 5, 2025 •

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