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Bug 1207843 - Part 3/3 - Clean up ARM Imm8::EncodeImm(). r=h4writer

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This commit is contained in:
Sean Stangl 2015-09-23 15:32:48 -07:00
parent 07561ec9ed
commit ea32fe0397
1 changed files with 6 additions and 26 deletions
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32
js/src/jit/arm/Assembler-arm.h
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@ -647,33 +647,13 @@ class Imm8 : public Operand2
public:
static datastore::Imm8mData EncodeImm(uint32_t imm) {
// mozilla::CountLeadingZeroes32(imm) requires imm != 0.
if (imm == 0)
return datastore::Imm8mData(0, 0);
int left = mozilla::CountLeadingZeroes32(imm) & 30;
// See if imm is a simple value that can be encoded with a rotate of 0.
// This is effectively imm <= 0xff, but I assume this can be optimized
// more.
if (left >= 24)
return datastore::Imm8mData(imm, 0);
// Mask out the 8 bits following the first bit that we found, see if we
// have 0 yet.
int no_imm = imm & ~(0xff << (24 - left));
if (no_imm == 0) {
return datastore::Imm8mData(imm >> (24 - left), ((8 + left) >> 1));
// An encodable integer has a maximum of 8 contiguous set bits,
// with an optional wrapped left rotation to even bit positions.
for (int rot = 0; rot < 16; rot++) {
uint32_t rotimm = mozilla::RotateLeft(imm, rot*2);
if (rotimm <= 0xFF)
return datastore::Imm8mData(rotimm, rot);
}
// Look for the most signifigant bit set, once again.
int right = 32 - (mozilla::CountLeadingZeroes32(no_imm) & 30);
// If it is in the bottom 8 bits, there is a chance that this is a
// wraparound case.
if (right >= 8)
return datastore::Imm8mData();
// Rather than masking out bits and checking for 0, just rotate the
// immediate that we were passed in, and see if it fits into 8 bits.
unsigned int mask = imm << (8 - right) | imm >> (24 + right);
if (mask <= 0xff)
return datastore::Imm8mData(mask, (8 - right) >> 1);
return datastore::Imm8mData();
}