rsqrtss

Reciprocal Square Root Scalar Single-Precision

RSQRTSS xmm1, xmm2/m32

Computes approximate reciprocal sqrt (1/sqrt(x)) of low float.

Details

Computes an approximate reciprocal square root (1/√x) of the low 32-bit single-precision floating-point value in the source operand and stores the result in the low 32 bits of the destination XMM register, while preserving the upper 96 bits of xmm1. No CPU flags are affected. The result has a maximum relative error of approximately 1.5×2^-12.

Pseudocode Operation

xmm1[0:31] ← 1.0 / sqrt(src[0:31]); xmm1[32:127] ← xmm1[32:127];

Example

RSQRTSS xmm1, xmm2/m32

Encoding

Binary Layout
F3
+0
0F
+1
52
+2
 
Format SSE
Opcode F3 0F 52
Extension SSE

Operands

  • dest
    128-bit XMM SIMD register
  • src
    128-bit XMM SIMD register or Memory operand

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
F3 0F 52 /r RSQRTSS xmm1, xmm2/m32 RM V/V SSE Computes the approximate reciprocal of the square root of the low single precision floating-point value in xmm2/m32 and stores the results in xmm1.
VEX.LIG.F3.0F.WIG 52 /r VRSQRTSS xmm1, xmm2, xmm3/m32 AVX RVM V/V Computes the approximate reciprocal of the square root of the low single precision floating-point value in xmm3/m32 and stores the results in xmm1. Also, upper single precision floating-point values (bits[127:32]) from xmm2 are copied to xmm1[127:32].

Description

Computes an approximate reciprocal of the square root of the low single precision floating-point value in the source operand (second operand) stores the single precision floating-point result in the destination operand. The source operand can be an XMM register or a 32-bit memory location. The destination operand is an XMM register. The three high-order doublewords of the destination operand remain unchanged. See Figure 10-6 in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, for an illustration of a scalar single precision floatingpoint operation. The relative error for this approximation is: |Relative Error| ≤ 1.5 ∗ 2−12 The RSQRTSS instruction is not affected by the rounding control bits in the MXCSR register. When a source value is a 0.0, an ∞ of the sign of the source value is returned. A denormal source value is treated as a 0.0 (of the same sign). When a source value is a negative value (other than −0.0), a floating-point indefinite is returned. When a source value is an SNaN or QNaN, the SNaN is converted to a QNaN or the source QNaN is returned. In 64-bit mode, using a REX prefix in the form of REX.R permits this instruction to access additional registers (XMM8-XMM15). 128-bit Legacy SSE version: The first source operand and the destination operand are the same. Bits (MAXVL1:32) of the corresponding YMM destination register remain unchanged. VEX.128 encoded version: Bits (MAXVL-1:128) of the destination YMM register are zeroed.

Operation

RSQRTSS (128-bit Legacy SSE Version)
DEST[31:0] := APPROXIMATE(1/SQRT(SRC2[31:0]))
DEST[MAXVL-1:32] (Unmodified)

VRSQRTSS (VEX.128 Encoded Version)
DEST[31:0] := APPROXIMATE(1/SQRT(SRC2[31:0]))
DEST[127:32] := SRC1[127:32]
DEST[MAXVL-1:128] := 0





RSQRTSS—Compute Reciprocal of Square Root of Scalar Single Precision Floating-Point Value                                                Vol. 2B 4-596

Intel C/C++ Compiler Intrinsic Equivalent

RSQRTSS __m128 _mm_rsqrt_ss(__m128 a)

Exceptions

SIMD Floating-Point Exceptions

None.

Other Exceptions

See Table 2-22, “Type 5 Class Exception Conditions.” RSQRTSS—Compute Reciprocal of Square Root of Scalar Single Precision Floating-Point Value Vol. 2B 4-597