diff --git a/packages/compiler/src/backend/llvm/dense-array-access.test.ts b/packages/compiler/src/backend/llvm/dense-array-access.test.ts
index f307fde54..92dac2afe 100644
--- a/packages/compiler/src/backend/llvm/dense-array-access.test.ts
+++ b/packages/compiler/src/backend/llvm/dense-array-access.test.ts
@@ -213,7 +213,7 @@ test("length assignments skip the runtime when the length is unchanged", () => {
     module("reset", VOID, [{ kind: "arraySetLength", arr: ref("s", words), length: index, loc }]),
   );
   const fn = body(llvm, "reset");
-  expect(fn).toMatch(/uitofp i64 .* to double/);
+  expect(fn).toMatch(/uitofp nneg i64 .* to double/);
   expect(fn).toMatch(/fcmp oeq double %p?\S*, %t\d+/);
   expect(fn.match(/call void @scr_arr_set_len\(/g)).toHaveLength(1);
   expect(fn).toMatch(/br i1 %t\d+, label %arr\.len\.done\d*, label %arr\.len\.set\d*/);
diff --git a/packages/compiler/src/backend/llvm/dense-array-access.ts b/packages/compiler/src/backend/llvm/dense-array-access.ts
index 88bea73ed..517658e94 100644
--- a/packages/compiler/src/backend/llvm/dense-array-access.ts
+++ b/packages/compiler/src/backend/llvm/dense-array-access.ts
@@ -411,7 +411,7 @@ export function emitDenseArrayPush(
     fastValue = B.tmp();
   B.line(`${next} = add nuw ${size} ${len}, 1`);
   B.line(`store ${size} ${next}, ptr ${lenPtr}${host.fieldAliasAttachment(lenPtr)}`);
-  B.line(`${fastValue} = uitofp ${size} ${next} to double`);
+  B.line(`${fastValue} = uitofp nneg ${size} ${next} to double`);
   B.br(join);
   B.startBlock(slow);
   const slowValue = B.tmp();
diff --git a/packages/compiler/src/backend/llvm/dyn.ts b/packages/compiler/src/backend/llvm/dyn.ts
index 86ac62d8d..5912020f9 100644
--- a/packages/compiler/src/backend/llvm/dyn.ts
+++ b/packages/compiler/src/backend/llvm/dyn.ts
@@ -1668,7 +1668,7 @@ export class LlDyn {
           host.declare(`declare void @scr_arr_set_len(ptr, double)`);
           const index = B.tmp();
           const length = B.tmp();
-          B.line(`${index} = uitofp ${this.S} ${i} to double`);
+          B.line(`${index} = uitofp nneg ${this.S} ${i} to double`);
           B.line(`${length} = fadd double ${index}, 1.0`);
           B.line(`call void @scr_arr_set_len(ptr ${a}, double ${length})`);
           B.br(doneLabel);
@@ -3409,7 +3409,7 @@ export class LlDyn {
         const n = this.lenOf(B, "%d");
         const nd = B.tmp();
         const r = B.tmp();
-        B.line(`${nd} = uitofp ${host.sizeType} ${n} to double`);
+        B.line(`${nd} = uitofp nneg ${host.sizeType} ${n} to double`);
         B.line(`${r} = call ptr @scr_dyn_new_num(double ${nd})`);
         B.terminate(`ret ptr ${r}`);
         B.startBlock(lStr);
@@ -3448,7 +3448,7 @@ export class LlDyn {
         const n2 = B.tmp();
         B.line(`${n2} = call double @scr_str_utf16_len(ptr ${s})`);
         const idxD = B.tmp();
-        B.line(`${idxD} = uitofp ${host.sizeType} ${idx} to double`);
+        B.line(`${idxD} = uitofp nneg ${host.sizeType} ${idx} to double`);
         const inR2 = B.tmp();
         B.line(`${inR2} = fcmp olt double ${idxD}, ${n2}`);
         const lHit2 = B.newLabel("kg.ash");
@@ -3599,7 +3599,7 @@ export class LlDyn {
       host.declare(`declare ptr @scr_dyn_jsval_iter_n(ptr, double)`);
       const count = B.tmp();
       const out = B.tmp();
-      B.line(`${count} = uitofp ${host.sizeType} %n to double`);
+      B.line(`${count} = uitofp nneg ${host.sizeType} %n to double`);
       B.line(`${out} = call ptr @scr_dyn_jsval_iter_n(ptr %d, double ${count})`);
       B.terminate(`ret ptr ${out}`);
       B.startBlock(lNotJv);
@@ -3737,7 +3737,7 @@ export class LlDyn {
         const index = B.tmp();
         const bd = B.tmp();
         const r = B.tmp();
-        B.line(`${index} = uitofp ${host.sizeType} ${i} to double`);
+        B.line(`${index} = uitofp nneg ${host.sizeType} ${i} to double`);
         B.line(`${bd} = call double @scr_bytes_get(ptr ${bts}, double ${index})`);
         B.line(`${r} = call ptr @scr_dyn_new_num(double ${bd})`);
         B.line(`store ptr ${r}, ptr ${itemSlot}`);
diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 8ce49a03d..8b9bb381d 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -1925,7 +1925,7 @@ export class LlEmitter {
         // The branch must dominate fptoui: NaN/out-of-range conversion
         // would produce poison. Fractional indices also need a fallback.
         `  %index = fptoui double %i to ${this.sizeType}`,
-        `  %roundtrip = uitofp ${this.sizeType} %index to double`,
+        `  %roundtrip = uitofp nneg ${this.sizeType} %index to double`,
         `  %integer = fcmp oeq double %roundtrip, %i`,
         `  br i1 %integer, label %read, label %fallback`,
         `read:`,
@@ -2759,14 +2759,14 @@ export class LlEmitter {
         `entry:`,
         `  %ndp = getelementptr inbounds %ScrMapIx, ptr %m, i32 0, i32 19`,
         `  %nd = load ${sz}, ptr %ndp`,
-        `  %ndf = uitofp ${sz} %nd to double`,
+        `  %ndf = uitofp nneg ${sz} %nd to double`,
         `  %lo = fcmp oge double %k, 0.0`,
         `  %hi = fcmp olt double %k, %ndf`,
         `  %in = and i1 %lo, %hi`,
         `  br i1 %in, label %direct, label %outside`,
         `direct:`,
         `  %i = fptoui double %k to ${sz}`,
-        `  %back = uitofp ${sz} %i to double`,
+        `  %back = uitofp nneg ${sz} %i to double`,
         `  %int = fcmp oeq double %back, %k`,
         `  br i1 %int, label %slot, label %miss`,
         `slot:`,
@@ -5626,7 +5626,7 @@ export class LlEmitter {
         B.line(`${lenPtr} = getelementptr inbounds %ScrArr, ptr ${arr.name}, i32 0, i32 1`);
         this.markMemoryPointer(lenPtr, "array:header");
         B.line(`${len} = load ${this.sizeType}, ptr ${lenPtr}${this.fieldAliasAttachment(lenPtr)}`);
-        B.line(`${lenNumber} = uitofp ${this.sizeType} ${len} to double`);
+        B.line(`${lenNumber} = uitofp nneg ${this.sizeType} ${len} to double`);
         B.line(`${same} = fcmp oeq double ${length.name}, ${lenNumber}`);
         B.condBr(same, done, resize);
         B.startBlock(resize);
@@ -6255,7 +6255,7 @@ export class LlEmitter {
           B.line(
             `${rawLen} = load ${this.sizeType}, ptr ${lenPtr}${this.fieldAliasAttachment(lenPtr)}`,
           );
-          B.line(`${len} = uitofp ${this.sizeType} ${rawLen} to double`);
+          B.line(`${len} = uitofp nneg ${this.sizeType} ${rawLen} to double`);
           B.line(`${inBounds} = fcmp olt double ${i}, ${len}`);
           cur = i;
         }
diff --git a/packages/compiler/src/backend/llvm/expr-bytes.ts b/packages/compiler/src/backend/llvm/expr-bytes.ts
index 21fd1ed32..30cea3f4f 100644
--- a/packages/compiler/src/backend/llvm/expr-bytes.ts
+++ b/packages/compiler/src/backend/llvm/expr-bytes.ts
@@ -122,7 +122,7 @@ export function emitBytesIndex(
   const nonnegative = B.tmp();
   const belowLen = B.tmp();
   const inRange = B.tmp();
-  B.line(`${lenF64} = uitofp ${host.sizeType} ${len} to double`);
+  B.line(`${lenF64} = uitofp nneg ${host.sizeType} ${len} to double`);
   B.line(`${nonnegative} = fcmp oge double ${index}, ${f64Lit(0)}`);
   B.line(`${belowLen} = fcmp olt double ${index}, ${lenF64}`);
   B.line(`${inRange} = and i1 ${nonnegative}, ${belowLen}`);
@@ -137,7 +137,7 @@ export function emitBytesIndex(
   const roundTrip = B.tmp();
   const integral = B.tmp();
   B.line(`${idx} = fptoui double ${index} to ${host.sizeType}`);
-  B.line(`${roundTrip} = uitofp ${host.sizeType} ${idx} to double`);
+  B.line(`${roundTrip} = uitofp nneg ${host.sizeType} ${idx} to double`);
   B.line(`${integral} = fcmp oeq double ${roundTrip}, ${index}`);
   B.condBr(integral, valid, invalid);
 
@@ -177,7 +177,7 @@ export function emitBytesLength(
   const count = bytes && size !== 1 ? B.tmp() : len;
   if (count !== len) B.line(`${count} = shl ${host.sizeType} ${len}, ${Math.log2(size)}`);
   const out = B.tmp();
-  B.line(`${out} = uitofp ${host.sizeType} ${count} to double`);
+  B.line(`${out} = uitofp nneg ${host.sizeType} ${count} to double`);
   return { name: out, type: F64 };
 }
 
diff --git a/packages/compiler/src/backend/llvm/expr-containers.ts b/packages/compiler/src/backend/llvm/expr-containers.ts
index 6e4886b0b..3cdce7c63 100644
--- a/packages/compiler/src/backend/llvm/expr-containers.ts
+++ b/packages/compiler/src/backend/llvm/expr-containers.ts
@@ -520,7 +520,7 @@ export function emitArrIntrinsic(
       B.line(`${p} = getelementptr inbounds %ScrArr, ptr ${r.name}, i32 0, i32 1`);
       host.markMemoryPointer(p, "array:header");
       B.line(`${len} = load ${host.sizeType}, ptr ${p}${host.fieldAliasAttachment(p)}`);
-      B.line(`${t} = uitofp ${host.sizeType} ${len} to double`);
+      B.line(`${t} = uitofp nneg ${host.sizeType} ${len} to double`);
       return { name: t, type: e.type };
     }
     case "getNumber": {
diff --git a/packages/compiler/src/backend/llvm/local-array-reads.ts b/packages/compiler/src/backend/llvm/local-array-reads.ts
index 6daeb1cd0..ac546e291 100644
--- a/packages/compiler/src/backend/llvm/local-array-reads.ts
+++ b/packages/compiler/src/backend/llvm/local-array-reads.ts
@@ -348,7 +348,7 @@ function emitDenseReferenceArrayRead(
   B.line(`${cap} = load ${host.sizeType}, ptr ${capPtr}${host.fieldAliasAttachment(capPtr)}`);
   if (integerIndex) B.line(`${inRange} = icmp ult ${host.sizeType} ${integerIndex}, ${cap}`);
   else {
-    B.line(`${capNumber} = uitofp ${host.sizeType} ${cap} to double`);
+    B.line(`${capNumber} = uitofp nneg ${host.sizeType} ${cap} to double`);
     B.line(`${nonnegative} = fcmp oge double ${index.name}, 0.0`);
     B.line(`${belowCap} = fcmp olt double ${index.name}, ${capNumber}`);
     B.line(`${inRange} = and i1 ${nonnegative}, ${belowCap}`);
@@ -361,7 +361,7 @@ function emitDenseReferenceArrayRead(
   if (integerIndex) B.br(dense);
   else {
     B.line(`${offset} = fptoui double ${index.name} to ${host.sizeType}`);
-    B.line(`${roundTrip} = uitofp ${host.sizeType} ${offset} to double`);
+    B.line(`${roundTrip} = uitofp nneg ${host.sizeType} ${offset} to double`);
     B.line(`${integral} = fcmp oeq double ${index.name}, ${roundTrip}`);
     B.condBr(integral, dense, slow);
   }
diff --git a/packages/compiler/src/backend/llvm/string-reads.ts b/packages/compiler/src/backend/llvm/string-reads.ts
index fe70d8a1b..24fa80dd9 100644
--- a/packages/compiler/src/backend/llvm/string-reads.ts
+++ b/packages/compiler/src/backend/llvm/string-reads.ts
@@ -45,7 +45,7 @@ export const STRING_READ_HELPERS: readonly StringReadHelper[] = [
       `  br i1 %ascii, label %fast, label %slow`,
       `fast:`,
       ...loadLength(sz),
-      `  %n = uitofp ${sz} %len to double`,
+      `  %n = uitofp nneg ${sz} %len to double`,
       `  ret double %n`,
       `slow:`,
       `  %r = call double @scr_str_utf16_len(ptr %s)`,
@@ -62,7 +62,7 @@ export const STRING_READ_HELPERS: readonly StringReadHelper[] = [
       `  br i1 %ascii, label %bounds, label %slow`,
       `bounds:`,
       ...loadLength(sz),
-      `  %lenf = uitofp ${sz} %len to double`,
+      `  %lenf = uitofp nneg ${sz} %len to double`,
       // NaN fails both ordered comparisons; -0 passes and truncates to 0.
       `  %lo = fcmp oge double %i, 0.0`,
       `  %hi = fcmp olt double %i, %lenf`,
diff --git a/packages/compiler/src/backend/llvm/string-slices.ts b/packages/compiler/src/backend/llvm/string-slices.ts
index dd373cfea..59c2962ab 100644
--- a/packages/compiler/src/backend/llvm/string-slices.ts
+++ b/packages/compiler/src/backend/llvm/string-slices.ts
@@ -97,7 +97,7 @@ export function emitStringSliceRead(host: LlvmEmitterContext, value: Slice): LlV
       `${pointer} = getelementptr inbounds { ${host.sizeType}, ${host.sizeType}, i32 }, ptr ${snapshot.range}, i32 0, i32 1`,
     );
     B.line(`${length} = load ${host.sizeType}, ptr ${pointer}`);
-    B.line(`${number} = uitofp ${host.sizeType} ${length} to double`);
+    B.line(`${number} = uitofp nneg ${host.sizeType} ${length} to double`);
     return { name: number, type: value.type };
   }
   if (value.method !== "charCodeAt") return null;