From 11eddafef8cea4ced9cc36a47bb511119320f8ff Mon Sep 17 00:00:00 2001
From: Dan Gohman <sunfish@mozilla.com>
Date: Mon, 12 Mar 2018 10:28:42 -0700
Subject: [PATCH] Avoid using floating-point values in expand_fcvt_to_sint.

Compute the bound values for expand_fcvt_to_sint using bitwise integer
arithmetic rather than floating-point arithmetic, to avoid relying on
host floating point arithmetic.
---
 lib/cretonne/src/ir/immediates.rs        | 40 ++++++++++++++++++++++++
 lib/cretonne/src/isa/intel/enc_tables.rs | 17 +++++++---
 2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/lib/cretonne/src/ir/immediates.rs b/lib/cretonne/src/ir/immediates.rs
index 2d3b1236ef..69c9aab51a 100644
--- a/lib/cretonne/src/ir/immediates.rs
+++ b/lib/cretonne/src/ir/immediates.rs
@@ -538,6 +538,17 @@ impl Ieee32 {
         Ieee32(exponent << t)
     }
 
+    /// Create an `Ieee32` number representing the greatest negative value
+    /// not convertable from f32 to a signed integer with width n.
+    pub fn fcvt_to_sint_negative_overflow<I: Into<i32>>(n: I) -> Ieee32 {
+        let n = n.into();
+        debug_assert!(n < 32);
+        debug_assert!(23 + 1 - n < 32);
+        Self::with_bits(
+            (1u32 << (32 - 1)) | Self::pow2(n - 1).0 | (1u32 << (23 + 1 - n)),
+        )
+    }
+
     /// Return self negated.
     pub fn neg(self) -> Ieee32 {
         Ieee32(self.0 ^ (1 << 31))
@@ -590,6 +601,17 @@ impl Ieee64 {
         Ieee64(exponent << t)
     }
 
+    /// Create an `Ieee64` number representing the greatest negative value
+    /// not convertable from f64 to a signed integer with width n.
+    pub fn fcvt_to_sint_negative_overflow<I: Into<i64>>(n: I) -> Ieee64 {
+        let n = n.into();
+        debug_assert!(n < 64);
+        debug_assert!(52 + 1 - n < 64);
+        Self::with_bits(
+            (1u64 << (64 - 1)) | Self::pow2(n - 1).0 | (1u64 << (52 + 1 - n)),
+        )
+    }
+
     /// Return self negated.
     pub fn neg(self) -> Ieee64 {
         Ieee64(self.0 ^ (1 << 63))
@@ -857,6 +879,15 @@ mod tests {
         assert_eq!(Ieee32::pow2(1).neg().to_string(), "-0x1.000000p1");
     }
 
+    #[test]
+    fn fcvt_to_sint_negative_overflow_ieee32() {
+        for n in &[8, 16] {
+            assert_eq!(-((1u32 << (n - 1)) as f32) - 1.0, unsafe {
+                mem::transmute(Ieee32::fcvt_to_sint_negative_overflow(*n))
+            });
+        }
+    }
+
     #[test]
     fn format_ieee64() {
         assert_eq!(Ieee64::with_float(0.0).to_string(), "0.0");
@@ -985,4 +1016,13 @@ mod tests {
 
         assert_eq!(Ieee64::pow2(1).neg().to_string(), "-0x1.0000000000000p1");
     }
+
+    #[test]
+    fn fcvt_to_sint_negative_overflow_ieee64() {
+        for n in &[8, 16, 32] {
+            assert_eq!(-((1u64 << (n - 1)) as f64) - 1.0, unsafe {
+                mem::transmute(Ieee64::fcvt_to_sint_negative_overflow(*n))
+            });
+        }
+    }
 }
diff --git a/lib/cretonne/src/isa/intel/enc_tables.rs b/lib/cretonne/src/isa/intel/enc_tables.rs
index 00c5ab99cb..d3f62adf7c 100644
--- a/lib/cretonne/src/isa/intel/enc_tables.rs
+++ b/lib/cretonne/src/isa/intel/enc_tables.rs
@@ -376,13 +376,22 @@ fn expand_fcvt_to_sint(
     let mut overflow_cc = FloatCC::LessThan;
     let output_bits = ty.lane_bits();
     let flimit = match xty {
-        ir::types::F32 => pos.ins().f32const(Ieee32::pow2(output_bits - 1).neg()),
+        // An f32 can represent `i16::min_value() - 1` exactly with precision to spare, so
+        // there are values less than -2^(N-1) that convert correctly to INT_MIN.
+        ir::types::F32 => {
+            pos.ins().f32const(if output_bits < 32 {
+                overflow_cc = FloatCC::LessThanOrEqual;
+                Ieee32::fcvt_to_sint_negative_overflow(output_bits)
+            } else {
+                Ieee32::pow2(output_bits - 1).neg()
+            })
+        }
         ir::types::F64 => {
             // An f64 can represent `i32::min_value() - 1` exactly with precision to spare, so
             // there are values less than -2^(N-1) that convert correctly to INT_MIN.
             pos.ins().f64const(if output_bits < 64 {
                 overflow_cc = FloatCC::LessThanOrEqual;
-                Ieee64::with_float(-((1u64 << (output_bits - 1)) as f64) - 1.0)
+                Ieee64::fcvt_to_sint_negative_overflow(output_bits)
             } else {
                 Ieee64::pow2(output_bits - 1).neg()
             })
@@ -394,8 +403,8 @@ fn expand_fcvt_to_sint(
 
     // Finally, we could have a positive value that is too large.
     let fzero = match xty {
-        ir::types::F32 => pos.ins().f32const(Ieee32::with_float(0.0)),
-        ir::types::F64 => pos.ins().f64const(Ieee64::with_float(0.0)),
+        ir::types::F32 => pos.ins().f32const(Ieee32::with_bits(0)),
+        ir::types::F64 => pos.ins().f64const(Ieee64::with_bits(0)),
         _ => panic!("Can't convert {}", xty),
     };
     let overflow = pos.ins().fcmp(FloatCC::GreaterThanOrEqual, x, fzero);