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| -rw-r--r-- | lib/Target/PowerPC/README.txt | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/lib/Target/PowerPC/README.txt b/lib/Target/PowerPC/README.txt index 938fff2865..149c2db34e 100644 --- a/lib/Target/PowerPC/README.txt +++ b/lib/Target/PowerPC/README.txt @@ -194,6 +194,59 @@ currently only commutes to avoid inserting a copy BEFORE the two addr instr. ===-------------------------------------------------------------------------=== +176.gcc contains a bunch of code like this (this occurs dozens of times): + +int %test(uint %mode.0.i.0) { + %tmp.79 = cast uint %mode.0.i.0 to sbyte ; <sbyte> [#uses=1] + %tmp.80 = cast sbyte %tmp.79 to int ; <int> [#uses=1] + %tmp.81 = shl int %tmp.80, ubyte 16 ; <int> [#uses=1] + %tmp.82 = and int %tmp.81, 16711680 + ret int %tmp.82 +} + +which we compile to: + +_test: + extsb r2, r3 + rlwinm r3, r2, 16, 8, 15 + blr + +The extsb is obviously dead. This can be handled by a future thing like +MaskedValueIsZero that checks to see if bits are ever demanded (in this case, +the sign bits are never used, so we can fold the sext_inreg to nothing). + +I'm seeing code like this: + + srwi r3, r3, 16 + extsb r3, r3 + rlwimi r4, r3, 16, 8, 15 + +in which the extsb is preventing the srwi from being nuked. + +===-------------------------------------------------------------------------=== + +Another example that occurs is: + +uint %test(int %specbits.6.1) { + %tmp.2540 = shr int %specbits.6.1, ubyte 11 ; <int> [#uses=1] + %tmp.2541 = cast int %tmp.2540 to uint ; <uint> [#uses=1] + %tmp.2542 = shl uint %tmp.2541, ubyte 13 ; <uint> [#uses=1] + %tmp.2543 = and uint %tmp.2542, 8192 ; <uint> [#uses=1] + ret uint %tmp.2543 +} + +which we codegen as: + +l1_test: + srawi r2, r3, 11 + rlwinm r3, r2, 13, 18, 18 + blr + +the srawi can be nuked by turning the SAR into a logical SHR (the sext bits are +dead), which I think can then be folded into the rlwinm. + +===-------------------------------------------------------------------------=== + Compile offsets from allocas: int *%test() { |