[libclc] [libclc] Fix exponent overflow and subnormal rounding in __clc_ldexp (PR #233823)

[Romaric Jodin via cfe-commits]

https://github.com/rjodinchr updated https://github.com/llvm/llvm-project/pull/233823

>From 0474d7b184987d0881c8e74bbece75265b468603 Mon Sep 17 00:00:00 2001
From: Romaric Jodin <rjodin at google.com>
Date: Tue, 20 Oct 2026 14:32:45 +0200
Subject: [PATCH 1/2] [libclc] Fix exponent overflow and subnormal rounding in
 __clc_ldexp

When fp32 subnormals are
supported (`!__clc_denormals_are_zero_fp32()`), `__clc_ldexp`
previously added `n` to the exponent with plain signed integer
addition, overflowing 32-bit `int` when `n` is near `INT_MAX` or
`INT_MIN` (e.g. `n = INT_MAX` wrapped around to `< -24` and flushed to
zero instead of returning infinity). Additionally, subnormal fp32
results were formed with a truncating right shift instead of
round-to-nearest-even, and shift counts were not clamped to valid
ranges.

Normalize subnormal fp32 inputs before adding `n`, use
`__clc_add_sat(exponent, n)` in both fp32 and fp64 `__clc_ldexp` to
prevent signed integer overflow, clamp shift counts, and apply
round-to-nearest-even when right-shifting into a subnormal fp32
result.

Change-Id: I98b3412d6fd383e292aacb6a63028319fe3fda53
---
 libclc/clc/lib/generic/math/clc_ldexp.cl | 37 +++++++++---------------
 1 file changed, 14 insertions(+), 23 deletions(-)

diff --git a/libclc/clc/lib/generic/math/clc_ldexp.cl b/libclc/clc/lib/generic/math/clc_ldexp.cl
index 59db94cf047fa8..d5b181445300f9 100644
--- a/libclc/clc/lib/generic/math/clc_ldexp.cl
+++ b/libclc/clc/lib/generic/math/clc_ldexp.cl
@@ -51,32 +51,23 @@ _CLC_DEF _CLC_OVERLOAD float __clc_ldexp(float x, int n) {
       127 -
       (__clc_as_uint((float)(__clc_as_float(val_ui | 0x3f800000) - 1.0f)) >>
        23);
-  int dexponent = 25 - fbh;
-  uint dval_ui = (((val_ui << fbh) & 0x007fffff) | (dexponent << 23));
-  int ex = dexponent + n - multiplier;
-  dexponent = ex;
-  uint val = sign | (ex << 23) | (dval_ui & 0x007fffff);
-  int ex1 = dexponent + multiplier;
-  ex1 = -ex1 + 25;
-  dval_ui = (((dval_ui & 0x007fffff) | 0x800000) >> ex1);
-  dval_ui = dexponent > 0 ? val : dval_ui;
-  dval_ui = dexponent > 254 ? 0x7f800000 : dval_ui; /*overflow*/
-  dval_ui = dexponent < -multiplier ? 0 : dval_ui;  /*underflow*/
-  dval_ui = dval_ui | sign;
-  val_f = __clc_as_float(dval_ui);
-
-  exponent += n;
-
-  val = sign | (exponent << 23) | (val_ui & 0x007fffff);
-  ex1 = exponent + multiplier;
-  ex1 = -ex1 + 25;
-  val_ui = (((val_ui & 0x007fffff) | 0x800000) >> ex1);
+  fbh = dexp == 0 ? __clc_clamp(fbh, 1, 23) : 0;
+  exponent = dexp == 0 ? 1 - fbh : exponent;
+  uint mant = (val_ui << fbh) & 0x007fffff;
+
+  exponent = __clc_add_sat(exponent, n);
+
+  uint val = sign | ((uint)exponent << 23) | mant;
+  int ex1 = __clc_clamp(1 - exponent, 1, 25);
+  uint full_mant = mant | 0x00800000;
+  val_ui = full_mant >> ex1;
+  uint guard = (full_mant >> (ex1 - 1)) & 1u;
+  uint sticky = (full_mant & ((1u << (ex1 - 1)) - 1u)) != 0u;
+  val_ui += guard & (sticky | (val_ui & 1u));
   val_ui = exponent > 0 ? val : val_ui;
   val_ui = exponent > 254 ? 0x7f800000 : val_ui; /*overflow*/
   val_ui = exponent < -multiplier ? 0 : val_ui;  /*underflow*/
   val_ui = val_ui | sign;
-
-  val_ui = dexp == 0 ? dval_ui : val_ui;
   val_f = __clc_as_float(val_ui);
 
   val_f = (__clc_isnan(x) || __clc_isinf(x) || val_x == 0) ? x : val_f;
@@ -99,7 +90,7 @@ _CLC_DEF _CLC_OVERLOAD double __clc_ldexp(double x, int n) {
 
   ux = c ? ux : l;
 
-  int v = e + n;
+  int v = __clc_add_sat(e, n);
   v = __clc_clamp(v, -0x7ff, 0x7ff);
 
   ux &= ~EXPBITS_DP64;

>From b5cf0a22dcb1a97e773d9340974a7da9bce9911b Mon Sep 17 00:00:00 2001
From: Romaric Jodin <rjodin at google.com>
Date: Fri, 23 Oct 2026 11:45:31 +0200
Subject: [PATCH 2/2] applying wenju-he feedback

---
 libclc/clc/lib/generic/math/clc_ldexp.cl | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/libclc/clc/lib/generic/math/clc_ldexp.cl b/libclc/clc/lib/generic/math/clc_ldexp.cl
index d5b181445300f9..dcd7ecdf48b6b8 100644
--- a/libclc/clc/lib/generic/math/clc_ldexp.cl
+++ b/libclc/clc/lib/generic/math/clc_ldexp.cl
@@ -60,10 +60,8 @@ _CLC_DEF _CLC_OVERLOAD float __clc_ldexp(float x, int n) {
   uint val = sign | ((uint)exponent << 23) | mant;
   int ex1 = __clc_clamp(1 - exponent, 1, 25);
   uint full_mant = mant | 0x00800000;
-  val_ui = full_mant >> ex1;
-  uint guard = (full_mant >> (ex1 - 1)) & 1u;
-  uint sticky = (full_mant & ((1u << (ex1 - 1)) - 1u)) != 0u;
-  val_ui += guard & (sticky | (val_ui & 1u));
+  val_ui =
+      (full_mant + ((1u << (ex1 - 1)) - 1u) + ((full_mant >> ex1) & 1u)) >> ex1;
   val_ui = exponent > 0 ? val : val_ui;
   val_ui = exponent > 254 ? 0x7f800000 : val_ui; /*overflow*/
   val_ui = exponent < -multiplier ? 0 : val_ui;  /*underflow*/