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WString: Optimize a bit #7553

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merged 4 commits into from
Nov 16, 2020
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WString: Optimize a bit #7553

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f2c7565
WString: Optimize a bit
jjsuwa-sys3175 Sep 9, 2020
6e76a3a
* optimize `init()` more
jjsuwa-sys3175 Nov 15, 2020
ebe4ceb
* revise the comment
jjsuwa-sys3175 Nov 15, 2020
df10fff
Merge branch 'master' into WString_cutWaste
d-a-v Nov 16, 2020
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16 changes: 11 additions & 5 deletions cores/esp8266/WString.h
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Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,7 @@ class String {
// if the initial value is null or invalid, or if memory allocation
// fails, the string will be marked as invalid (i.e. "if (s)" will
// be false).
String() {
String() __attribute__((always_inline)) { // See init()
init();
}
String(const char *cstr);
Expand Down Expand Up @@ -323,10 +323,16 @@ class String {
char *wbuffer() const { return isSSO() ? const_cast<char *>(sso.buff) : ptr.buff; } // Writable version of buffer

protected:
void init(void) {
sso.buff[0] = 0;
sso.len = 0;
sso.isHeap = 0;
void init(void) __attribute__((always_inline)) {
sso.buff[0] = 0; // In the Xtensa ISA, in fact, 32-bit store insn ("S32I.N") is one-byte shorter than 8-bit one ("S8I").
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@TD-er TD-er Nov 15, 2020 •
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About the comment; One byte or one bit shorter?
Or do you mean the generated binary code?

sso.buff[1] = 0; // Thanks to store-merging optimization, these 9 lines emit only 3 insns:
sso.buff[2] = 0; // "MOVI.N aX,0", "S32I.N aX,a2,0" and "S32I.N aX,a2,8" (6 bytes in total)
sso.buff[3] = 0;
sso.buff[8] = 0; // Unfortunately, GCC seems not to re-evaluate the cost of inlining after the store-merging optimizer stage,
sso.buff[9] = 0; // `always_inline` attribute is necessary in order to assure inlining.
sso.buff[10] = 0;
sso.len = 0;
sso.isHeap = 0;
}
void invalidate(void);
unsigned char changeBuffer(unsigned int maxStrLen);
Expand Down