prim-bool.md

@@ -171,10 +171,63 @@ case (x <$# 0#) `orI#` (x >=$# width) `orI#` (y <$# 0#) `orI#` (y >=$# height) o
 ```
 
 
-Using the LLVM backend this compiles to:
+Let's analyze how that code gets compiled by the LLVM backend. For purposes of this analysis I will convert the above case expression to the following function:
+ 
+
+```wiki
+f :: Int# -> Int# -> Int# -> Int# -> String
+f x y width height =
+    case (x <$# 0#) `orI#` (x >=$# width) `orI#` (y <$# 0#) `orI#` (y >=$# height) of
+      1# -> "one"
+      0# -> "zero"
+```
+
+
+By dumping intermediate Cmm representation we can determine how variables are mapped to CPU registers. Arguments are passed to the function using a stack:
+
+```wiki
+Main.f_slow() //  [R1]
+         { info_tbl: []
+           stack_info: arg_space: 0 updfr_space: Nothing
+         }
+     {offset
+       cWY:
+           R5 = I64[Sp + 24];
+           R4 = I64[Sp + 16];
+           R3 = I64[Sp + 8];
+           R2 = I64[Sp];
+           R1 = R1;
+           Sp = Sp + 32;
+           call Main.f_info(R5, R4, R3, R2, R1) args: 8, res: 0, upd: 8;
+     }
+ }
+```
+
+`R2` contains the first argument `x`, `R3` contains `y`, `R4` contains `width` and `R5` contains `height`. We can verify that by looking at the body of `Main.f_info`:
 
 ```wiki
-# BB#0:                                 # %c1oe
+_sV9::I64 = R3;
+_sV3::I64 = R2;
+_sWx::I64 = %MO_S_Lt_W64(_sV3::I64, 0) | %MO_S_Ge_W64(_sV3::I64, R4) |
+            %MO_S_Lt_W64(_sV9::I64, 0) | %MO_S_Ge_W64(_sV9::I64, R5);
+```
+
+
+Mappings between Cmm's `R(2/3/4/5)` registers and machine registers are defined in `includes/stg/MachRegs.h`:
+
+```wiki
+#define REG_R2 r14
+#define REG_R3 rsi
+#define REG_R4 rdi
+#define REG_R5 r8
+```
+
+
+Knowing that we can dump the assembly generated by LLVM backend:
+
+```wiki
+Main_f_info:
+# BB#0:
    movq    %rsi, %rax
    orq     %r14, %rax
    shrq    $63, %rax
@@ -186,23 +239,55 @@ Using the LLVM backend this compiles to:
    setge   %al
    movzbl  %al, %eax
    orq     %rcx, %rax
-  jne    .LBB2_1
-# BB#3:                                 # %c1oP
-  movq   (%rbp), %rax
-  movl   $r1mu_closure+1, %ebx
+   jne     .LBB4_1
+# BB#3:
+   movq    Main_f2_closure(%rip), %rax
+   movl    $Main_f2_closure, %ebx
    jmpq    *%rax  # TAILCALL
-.LBB2_1:                                # %c1oe
+.LBB4_1:
    cmpq    $1, %rax
-  jne    .LBB2_2
-# BB#4:                                 # %c1oZ
-  movq   (%rbp), %rax
-  movl   $r1mv_closure+1, %ebx
+   jne     .LBB4_2
+# BB#4:
+   movq    Main_f1_closure(%rip), %rax
+   movl    $Main_f1_closure, %ebx
    jmpq    *%rax  # TAILCALL
-.LBB2_2:                                # %c1oF
-  movq   r1mt_closure(%rip), %rax
-  movl   $r1mt_closure, %ebx
+.LBB4_2:
+   movq    Main_f3_closure(%rip), %rax
+   movl    $Main_f3_closure, %ebx
    jmpq    *%rax  # TAILCALL
+```
 
+
+Let's analyze line by line the part responsible for evaluating the scrutinee:
+
+```wiki
+  movq   %rsi, %rax   # load y (stored in %rsi) into %rax register
+  orq    %r14, %rax   # perform bitwise OR between y (now in %rax) and x (in %r14)
+                      # and store result in %rax
+  shrq   $63, %rax    # shift %rax 63 bits to the right. If both x and y were
+                      # positive numbers, then their sign bits (MSB) were set to
+                      # 0 and so %rax is now 0. If at least one of them was
+                      # negative then its sign bit must have been 1 and so %rax
+                      # is now 1
+  cmpq   %rdi, %r14   # compare width (in %rdi) with x (in %r14) and set the flags
+                      # in the flag register according to the result.
+  setge  %cl          # if, based on flags set by cmpq, x was greater or equal to 
+                      # width then we set %cl to 1. Otherwise it is set to 0.
+  movzbl %cl, %ecx    # zero the bits of %ecx register except the lowest 8 bits
+                      # containg the result of previous operation
+  orq    %rax, %rcx   # perform logical OR on results of the previous test and
+                      # store the result in %rcx. At this point if %rcx is 1
+                      # then either x was negative, or y was negative or
+                      # x was greater or equal to width
+  cmpq   %r8, %rsi    # now we are checking whether y is greter or equal to
+                      # height. This is the same as previosly for x and width.
+  setge  %al          # This time we set LSB of %al to 1 if y >= height
+  movzbl %al, %eax    # as previously, clear bits of %eax except lowest 8 bits
+  orq    %rcx, %rax   # perform logical OR which combines result of previous
+                      # three comparisons with the last one    
+  jne    .LBB2_1      # if ZF is not set it this means that either %rcx or %rax
+                      # was not zero, which means that at least one condition
+                      # in the scrutinee was true
 ```