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| -rw-r--r-- | lib/Target/README.txt | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 70dc136..17617ad 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -810,6 +810,51 @@ badness. PPC64 misses f, f5 and f6. CellSPU aborts in isel. //===---------------------------------------------------------------------===// +This (and similar related idioms): + +unsigned int foo(unsigned char i) { + return i | (i<<8) | (i<<16) | (i<<24); +} + +compiles into: + +define i32 @foo(i8 zeroext %i) nounwind readnone ssp noredzone { +entry: + %conv = zext i8 %i to i32 + %shl = shl i32 %conv, 8 + %shl5 = shl i32 %conv, 16 + %shl9 = shl i32 %conv, 24 + %or = or i32 %shl9, %conv + %or6 = or i32 %or, %shl5 + %or10 = or i32 %or6, %shl + ret i32 %or10 +} + +it would be better as: + +unsigned int bar(unsigned char i) { + unsigned int j=i | (i << 8); + return j | (j<<16); +} + +aka: + +define i32 @bar(i8 zeroext %i) nounwind readnone ssp noredzone { +entry: + %conv = zext i8 %i to i32 + %shl = shl i32 %conv, 8 + %or = or i32 %shl, %conv + %shl5 = shl i32 %or, 16 + %or6 = or i32 %shl5, %or + ret i32 %or6 +} + +or even i*0x01010101, depending on the speed of the multiplier. The best way to +handle this is to canonicalize it to a multiply in IR and have codegen handle +lowering multiplies to shifts on cpus where shifts are faster. + +//===---------------------------------------------------------------------===// + We do a number of simplifications in simplify libcalls to strength reduce standard library functions, but we don't currently merge them together. For example, it is useful to merge memcpy(a,b,strlen(b)) -> strcpy. This can only |