transpile: Flip comparison operators when negating in CFG (#1773)

ahomescu · web-flow · commit 0bcaa7402ece · 2026-05-22T15:54:48.000-07:00
Produces cleaner code, with relatively small changes.
diff --git a/c2rust-transpile/src/cfg/structures.rs b/c2rust-transpile/src/cfg/structures.rs
@@ -2,7 +2,7 @@
 
 use super::*;
 use log::warn;
-use syn::{spanned::Spanned as _, ExprBreak, ExprIf, ExprUnary, Stmt};
+use syn::{spanned::Spanned as _, ExprBinary, ExprBreak, ExprIf, ExprUnary, Stmt};
 
 use crate::rust_ast::{comment_store, set_span::SetSpan, BytePos, SpanExt};
 
@@ -1048,15 +1048,33 @@ impl StructureState {
 /// Take the logical negation of an expression.
 ///
 ///   * Negating something of the form `!<expr>` produces `<expr>`
+///   * Negating a comparison operator produces the opposite operator.
 ///
 fn not(bool_expr: &Expr) -> Box<Expr> {
-    use syn::UnOp;
+    use syn::{BinOp::*, UnOp};
     match *bool_expr {
         Expr::Unary(ExprUnary {
             op: UnOp::Not(_),
             ref expr,
             ..
         }) => Box::new(unparen(expr).clone()),
+        Expr::Binary(ExprBinary {
+            ref left,
+            op: op @ (Eq(_) | Ne(_) | Gt(_) | Lt(_) | Ge(_) | Le(_)),
+            ref right,
+            ..
+        }) => {
+            let op = match op {
+                Eq(_) => Ne(Default::default()),
+                Ne(_) => Eq(Default::default()),
+                Gt(_) => Le(Default::default()),
+                Lt(_) => Ge(Default::default()),
+                Ge(_) => Lt(Default::default()),
+                Le(_) => Gt(Default::default()),
+                _ => unreachable!(),
+            };
+            mk().binary_expr(op, left.clone(), right.clone())
+        }
         _ => mk().unary_expr(UnOp::Not(Default::default()), Box::new(bool_expr.clone())),
     }
 }
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile-linux@irreducible.c.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile-linux@irreducible.c.snap
@@ -32,7 +32,7 @@ pub unsafe extern "C" fn irreducible(mut x: ::core::ffi::c_int) -> ::core::ffi::
                     }
                 }
                 _ => {
-                    if !(x < 20 as ::core::ffi::c_int) {
+                    if x >= 20 as ::core::ffi::c_int {
                         break 's_36;
                     }
                     x += 90 as ::core::ffi::c_int;
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile-macos@irreducible.c.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile-macos@irreducible.c.snap
@@ -32,7 +32,7 @@ pub unsafe extern "C" fn irreducible(mut x: ::core::ffi::c_int) -> ::core::ffi::
                     }
                 }
                 _ => {
-                    if !(x < 20 as ::core::ffi::c_int) {
+                    if x >= 20 as ::core::ffi::c_int {
                         break 's_36;
                     }
                     x += 90 as ::core::ffi::c_int;
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@gotos.c.2021.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@gotos.c.2021.clang15.snap
@@ -14,7 +14,7 @@ expression: cat tests/snapshots/gotos.2021.clang15.rs
 #[no_mangle]
 pub unsafe extern "C" fn sum(mut count: ::core::ffi::c_int) -> ::core::ffi::c_int {
     let mut x: ::core::ffi::c_int = 0 as ::core::ffi::c_int;
-    while !(count <= 0 as ::core::ffi::c_int) {
+    while count > 0 as ::core::ffi::c_int {
         x += count;
         count -= 1;
     }
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@gotos.c.2024.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@gotos.c.2024.clang15.snap
@@ -15,7 +15,7 @@ expression: cat tests/snapshots/gotos.2024.clang15.rs
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn sum(mut count: ::core::ffi::c_int) -> ::core::ffi::c_int {
     let mut x: ::core::ffi::c_int = 0 as ::core::ffi::c_int;
-    while !(count <= 0 as ::core::ffi::c_int) {
+    while count > 0 as ::core::ffi::c_int {
         x += count;
         count -= 1;
     }
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@lift_const.c.2021.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@lift_const.c.2021.clang15.snap
@@ -20,7 +20,7 @@ pub unsafe extern "C" fn lift_const(
     mut argv: *mut *mut ::core::ffi::c_char,
 ) {
     let mut starts_with_a: ::core::ffi::c_int = 0;
-    if !(argc == 3 as ::core::ffi::c_int) {
+    if argc != 3 as ::core::ffi::c_int {
         starts_with_a = (**argv.offset(0 as ::core::ffi::c_int as isize) as ::core::ffi::c_int
             == 'a' as ::core::ffi::c_int) as ::core::ffi::c_int;
         if starts_with_a != 0 {
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@lift_const.c.2024.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@lift_const.c.2024.clang15.snap
@@ -21,7 +21,7 @@ pub unsafe extern "C" fn lift_const(
     mut argv: *mut *mut ::core::ffi::c_char,
 ) {
     let mut starts_with_a: ::core::ffi::c_int = 0;
-    if !(argc == 3 as ::core::ffi::c_int) {
+    if argc != 3 as ::core::ffi::c_int {
         starts_with_a = (**argv.offset(0 as ::core::ffi::c_int as isize) as ::core::ffi::c_int
             == 'a' as ::core::ffi::c_int) as ::core::ffi::c_int;
         if starts_with_a != 0 {
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@loops.c.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@loops.c.snap
@@ -38,7 +38,7 @@ pub unsafe extern "C" fn infinite_loop(mut x: ::core::ffi::c_int) {
 pub unsafe extern "C" fn goto_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_int {
     loop {
         x += 1 as ::core::ffi::c_int;
-        if !(x != 0) {
+        if x == 0 {
             break;
         }
     }
@@ -48,7 +48,7 @@ pub unsafe extern "C" fn goto_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_
 pub unsafe extern "C" fn do_while_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_int {
     loop {
         x += 1 as ::core::ffi::c_int;
-        if !(x != 0) {
+        if x == 0 {
             break;
         }
     }
@@ -206,7 +206,7 @@ pub unsafe extern "C" fn do_loop_single_continue(mut x: ::core::ffi::c_int) -> :
             }
             x += 1 as ::core::ffi::c_int;
         }
-        if !(x != 0) {
+        if x == 0 {
             break;
         }
     }
@@ -229,7 +229,7 @@ pub unsafe extern "C" fn do_loop_multi_continue(mut x: ::core::ffi::c_int) -> ::
             }
             x += 2 as ::core::ffi::c_int;
         }
-        if !(x != 0) {
+        if x == 0 {
             break;
         }
     }
@@ -354,14 +354,14 @@ pub unsafe extern "C" fn goto_loops(mut x: ::core::ffi::c_int) -> ::core::ffi::c
         if x != 0 {
             loop {
                 x += 1 as ::core::ffi::c_int;
-                if !(x != 0) {
+                if x == 0 {
                     break '_D;
                 }
             }
         } else {
             loop {
                 x += 2 as ::core::ffi::c_int;
-                if !(x != 0) {
+                if x == 0 {
                     break '_D;
                 }
             }
@@ -375,14 +375,14 @@ pub unsafe extern "C" fn goto_into_loops(mut x: ::core::ffi::c_int) {
         if x != 0 {
             loop {
                 x += 1 as ::core::ffi::c_int;
-                if !(x != 0) {
+                if x == 0 {
                     break '_D;
                 }
             }
         } else {
             loop {
                 x += 2 as ::core::ffi::c_int;
-                if !(x != 0) {
+                if x == 0 {
                     break '_D;
                 }
             }
@@ -394,14 +394,14 @@ pub unsafe extern "C" fn goto_separate_loops(mut x: ::core::ffi::c_int) {
     if x != 0 {
         loop {
             x += 1 as ::core::ffi::c_int;
-            if !(x != 0) {
+            if x == 0 {
                 break;
             }
         }
     } else {
         loop {
             x += 2 as ::core::ffi::c_int;
-            if !(x != 0) {
+            if x == 0 {
                 break;
             }
         }
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@varargs.c.2021.x86_64.linux.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@varargs.c.2021.x86_64.linux.clang15.snap
@@ -68,7 +68,7 @@ pub unsafe extern "C" fn my_printf(mut fmt: *const ::core::ffi::c_char, mut c2ru
         match *fmt as ::core::ffi::c_int {
             37 => {
                 fmt = fmt.offset(1);
-                if !(*fmt == 0) {
+                if *fmt != 0 {
                     match *fmt as ::core::ffi::c_int {
                         105 | 100 => {
                             printf(
diff --git a/c2rust-transpile/tests/snapshots/snapshots__transpile@varargs.c.2024.x86_64.linux.clang15.snap b/c2rust-transpile/tests/snapshots/snapshots__transpile@varargs.c.2024.x86_64.linux.clang15.snap
@@ -69,7 +69,7 @@ pub unsafe extern "C" fn my_printf(mut fmt: *const ::core::ffi::c_char, mut c2ru
         match *fmt as ::core::ffi::c_int {
             37 => {
                 fmt = fmt.offset(1);
-                if !(*fmt == 0) {
+                if *fmt != 0 {
                     match *fmt as ::core::ffi::c_int {
                         105 | 100 => {
                             printf(

Original file line number	Diff line number	Diff line change
`@@ -32,7 +32,7 @@ pub unsafe extern "C" fn irreducible(mut x: ::core::ffi::c_int) -> ::core::ffi::`
`32`	`32`	`}`
`33`	`33`	`}`
`34`	`34`	`_ => {`
`35`		`- if !(x < 20 as ::core::ffi::c_int) {`
	`35`	`+ if x >= 20 as ::core::ffi::c_int {`
`36`	`36`	`break 's_36;`
`37`	`37`	`}`
`38`	`38`	`x += 90 as ::core::ffi::c_int;`
Original file line number	Diff line number	Diff line change
`@@ -14,7 +14,7 @@ expression: cat tests/snapshots/gotos.2021.clang15.rs`
`14`	`14`	`#[no_mangle]`
`15`	`15`	`pub unsafe extern "C" fn sum(mut count: ::core::ffi::c_int) -> ::core::ffi::c_int {`
`16`	`16`	`let mut x: ::core::ffi::c_int = 0 as ::core::ffi::c_int;`
`17`		`- while !(count <= 0 as ::core::ffi::c_int) {`
	`17`	`+ while count > 0 as ::core::ffi::c_int {`
`18`	`18`	`x += count;`
`19`	`19`	`count -= 1;`
`20`	`20`	`}`
Original file line number	Diff line number	Diff line change
`@@ -15,7 +15,7 @@ expression: cat tests/snapshots/gotos.2024.clang15.rs`
`15`	`15`	`#[unsafe(no_mangle)]`
`16`	`16`	`pub unsafe extern "C" fn sum(mut count: ::core::ffi::c_int) -> ::core::ffi::c_int {`
`17`	`17`	`let mut x: ::core::ffi::c_int = 0 as ::core::ffi::c_int;`
`18`		`- while !(count <= 0 as ::core::ffi::c_int) {`
	`18`	`+ while count > 0 as ::core::ffi::c_int {`
`19`	`19`	`x += count;`
`20`	`20`	`count -= 1;`
`21`	`21`	`}`
Original file line number	Diff line number	Diff line change
`@@ -38,7 +38,7 @@ pub unsafe extern "C" fn infinite_loop(mut x: ::core::ffi::c_int) {`
`38`	`38`	`pub unsafe extern "C" fn goto_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_int {`
`39`	`39`	`loop {`
`40`	`40`	`x += 1 as ::core::ffi::c_int;`
`41`		`- if !(x != 0) {`
	`41`	`+ if x == 0 {`
`42`	`42`	`break;`
`43`	`43`	`}`
`44`	`44`	`}`
`@@ -48,7 +48,7 @@ pub unsafe extern "C" fn goto_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_`
`48`	`48`	`pub unsafe extern "C" fn do_while_loop(mut x: ::core::ffi::c_int) -> ::core::ffi::c_int {`
`49`	`49`	`loop {`
`50`	`50`	`x += 1 as ::core::ffi::c_int;`
`51`		`- if !(x != 0) {`
	`51`	`+ if x == 0 {`
`52`	`52`	`break;`
`53`	`53`	`}`
`54`	`54`	`}`
`@@ -206,7 +206,7 @@ pub unsafe extern "C" fn do_loop_single_continue(mut x: ::core::ffi::c_int) -> :`
`206`	`206`	`}`
`207`	`207`	`x += 1 as ::core::ffi::c_int;`
`208`	`208`	`}`
`209`		`- if !(x != 0) {`
	`209`	`+ if x == 0 {`
`210`	`210`	`break;`
`211`	`211`	`}`
`212`	`212`	`}`
`@@ -229,7 +229,7 @@ pub unsafe extern "C" fn do_loop_multi_continue(mut x: ::core::ffi::c_int) -> ::`
`229`	`229`	`}`
`230`	`230`	`x += 2 as ::core::ffi::c_int;`
`231`	`231`	`}`
`232`		`- if !(x != 0) {`
	`232`	`+ if x == 0 {`
`233`	`233`	`break;`
`234`	`234`	`}`
`235`	`235`	`}`
`@@ -354,14 +354,14 @@ pub unsafe extern "C" fn goto_loops(mut x: ::core::ffi::c_int) -> ::core::ffi::c`
`354`	`354`	`if x != 0 {`
`355`	`355`	`loop {`
`356`	`356`	`x += 1 as ::core::ffi::c_int;`
`357`		`- if !(x != 0) {`
	`357`	`+ if x == 0 {`
`358`	`358`	`break '_D;`
`359`	`359`	`}`
`360`	`360`	`}`
`361`	`361`	`} else {`
`362`	`362`	`loop {`
`363`	`363`	`x += 2 as ::core::ffi::c_int;`
`364`		`- if !(x != 0) {`
	`364`	`+ if x == 0 {`
`365`	`365`	`break '_D;`
`366`	`366`	`}`
`367`	`367`	`}`
`@@ -375,14 +375,14 @@ pub unsafe extern "C" fn goto_into_loops(mut x: ::core::ffi::c_int) {`
`375`	`375`	`if x != 0 {`
`376`	`376`	`loop {`
`377`	`377`	`x += 1 as ::core::ffi::c_int;`
`378`		`- if !(x != 0) {`
	`378`	`+ if x == 0 {`
`379`	`379`	`break '_D;`
`380`	`380`	`}`
`381`	`381`	`}`
`382`	`382`	`} else {`
`383`	`383`	`loop {`
`384`	`384`	`x += 2 as ::core::ffi::c_int;`
`385`		`- if !(x != 0) {`
	`385`	`+ if x == 0 {`
`386`	`386`	`break '_D;`
`387`	`387`	`}`
`388`	`388`	`}`
`@@ -394,14 +394,14 @@ pub unsafe extern "C" fn goto_separate_loops(mut x: ::core::ffi::c_int) {`
`394`	`394`	`if x != 0 {`
`395`	`395`	`loop {`
`396`	`396`	`x += 1 as ::core::ffi::c_int;`
`397`		`- if !(x != 0) {`
	`397`	`+ if x == 0 {`
`398`	`398`	`break;`
`399`	`399`	`}`
`400`	`400`	`}`
`401`	`401`	`} else {`
`402`	`402`	`loop {`
`403`	`403`	`x += 2 as ::core::ffi::c_int;`
`404`		`- if !(x != 0) {`
	`404`	`+ if x == 0 {`
`405`	`405`	`break;`
`406`	`406`	`}`
`407`	`407`	`}`