MAXSS

Return Maximum Scalar Single Precision Floating-Point Value

stableVMJITAOTinstruction

Encodings

Operand encoding

Each mode is a value of the Op/En column above. It says which field of the encoded instruction carries each operand, in the order they are written, and whether the instruction reads it, writes it or both.

Measured cost

Description#

Compares the low single precision floating-point values in the first source operand and the second source operand, and returns the maximum value to the low doubleword of the destination operand.

If the values being compared are both 0.0s (of either sign), the value in the second source operand is returned. If a value in the second source operand is an SNaN, that SNaN is returned unchanged to the destination (that is, a QNaN version of the SNaN is not returned).

If only one value is a NaN (SNaN or QNaN) for this instruction, the second source operand, either a NaN or a valid floating-point value, is written to the result. If instead of this behavior, it is required that the NaN from either source operand be returned, the action of MAXSS can be emulated using a sequence of instructions, such as, a comparison followed by AND, ANDN, and OR.

The second source operand can be an XMM register or a 32-bit memory location. The first source and destination operands are XMM registers.

128-bit Legacy SSE version: The destination and first source operand are the same. Bits (MAXVL:32) of the corresponding destination register remain unchanged.

VEX.128 and EVEX encoded version: The first source operand is an xmm register encoded by VEX.vvvv. Bits (127:32) of the XMM register destination are copied from corresponding bits in the first source operand. Bits (MAXVL:128) of the destination register are zeroed.

EVEX encoded version: The low doubleword element of the destination operand is updated according to the writemask.

Software should ensure VMAXSS is encoded with VEX.L=0. Encoding VMAXSS with VEX.L=1 may encounter unpredictable behavior across different processor generations.

Operation#

MAX(SRC1, SRC2)
{

    IF ((SRC1 = 0.0) and (SRC2 = 0.0)) THEN DEST := SRC2;
          ELSE IF (SRC1 = NaN) THEN DEST := SRC2; FI;
          ELSE IF (SRC2 = NaN) THEN DEST := SRC2; FI;
          ELSE IF (SRC1 > SRC2) THEN DEST := SRC1;
          ELSE DEST := SRC2;

    FI;
}

VMAXSS (EVEX Encoded Version)

IF k1[0] or *no writemask*

     THEN DEST[31:0] := MAX(SRC1[31:0], SRC2[31:0])

     ELSE

     IF *merging-masking*                  ; merging-masking

           THEN *DEST[31:0] remains unchanged*

           ELSE                            ; zeroing-masking

           THEN DEST[31:0] := 0

     FI;

FI;

DEST[127:32] := SRC1[127:32]

DEST[MAXVL-1:128] := 0

VMAXSS (VEX.128 Encoded Version)
DEST[31:0] := MAX(SRC1[31:0], SRC2[31:0])
DEST[127:32] := SRC1[127:32]
DEST[MAXVL-1:128] := 0

MAXSS (128-bit Legacy SSE Version)
DEST[31:0] := MAX(DEST[31:0], SRC[31:0])
DEST[MAXVL-1:32] (Unmodified)

Intel C/C++ compiler intrinsics#

VMAXSS __m128 _mm_max_round_ss( __m128 a, __m128 b, int);
VMAXSS __m128 _mm_mask_max_round_ss(__m128 s, __mmask8 k, __m128 a, __m128 b, int);
VMAXSS __m128 _mm_maskz_max_round_ss( __mmask8 k, __m128 a, __m128 b, int);
MAXSS __m128 _mm_max_ss(__m128 a, __m128 b);

SIMD Floating-Point Exceptions#

Invalid (Including QNaN Source Operand), Denormal.

Other Exceptions#

Non-EVEX-encoded instruction, see Table 2-20, "Type 3 Class Exception Conditions." EVEX-encoded instruction, see Table 2-49, "Type E3 Class Exception Conditions."

Sources

• This instruction in the Intel manual
• Latency and throughput per microarchitecture